Main Guidelines for Quests Development

Quests should primarily provide useful information to the player and guide them through the pack progression. But feel free to develop some lore on top of that.
Everybody is welcome to contribute to the questbook. But any bigger changes need to be discussed in the quest-dev channel on discord first.
You should only change quests on topics you have expertise on and have actually played around with yourself. The only exception to this are mechanics/items/machines that are brand-new additions, in which case you should talk to the responsible dev or study the PR that added these things. Ideally get that dev to write the quest or at least help you with it.

Some more Guidelines for Quests Development

Secret quests should only see very limited use for surprise bonuses or riddles.
Crafting quests at the start of the game will stay (this has been discussed on several occasions with Dreammaster). But crafting quests should not be used elsewhere.
Late game quests should ideally not have mandatory tasks for fluid cells. That is not how players craft things at that stage. There can be exceptions.
Most quests should only be in one quest line. However, there are important exceptions. Specifically, the Tips and Tricks quest line and transition quests that lead you from one tab to another. There can be other cases where it makes sense.
Not every quest ticket on github is actually correct. Check things yourself before changing a quest.
The main colors used in quest descriptions, and their associated tags, are red for warnings [warn], aqua for notes [note] and blue for hyperlinks [url]. The colors and other formatting will be automatically applied when you use these tags, with lighter colors for dark themes and darker colors for bright themes, for easier reading. Use [warn]text here[/warn] and potentially add "NOTE:" or "WARNING:" before important information and warnings. Use [note]text here[/note] for a PS at the end of a quest which can include talk about progression, alternatives, or other meta comments like the quest authors opinion on things. Regular advice should ideally be in default color as part of the text though.
You can add working links to websites in a quest description. To do so, please use the format [url]https://wiki.gtnewhorizons.com[/url]. That makes the link functional, turns it blue, and underlines it. Of course you can link elsewhere besides the wiki as necessary.

Git Basics

This is not a Git tutorial. Familiarize yourself a bit with Git before trying to do quest updates. https://git-scm.com/docs/gittutorial https://try.github.io/
As an alternate to using Git from the command line, you can use GitHub Desktop as a GUI. It makes things very simple, and you don't need to remember any commands. We will focus on that option in the following.

Git Definitions

Repo - The database of all the files and their history. The remote repo is on Github. The local repo is on your drive. They do not automatically track each other.
Source/upstream repo - The original/parent/upstream repo, in this case GTNewHorizons/GT-New-Horizons-Modpack
Forked repo - Your personal repo, for example chochem/GT-New-Horizons-Modpack
Local repo - The copy of a repo on your local PC
origin repo - The copy of a repo out on github
Commit - A bundle of changes that are added to the repo. Each is assigned a unique SHA tag. Initially commits are stored in your local repo.
Branch - A branch represents an independent line of development. Master is typically the "primary" branch. Any new branch is started off a particular state of an old one. This starting point can also come from a branch in the upstream repo (e.g. the up to date master in the upstream repo!). From there the branch is then independent.
Fetch - An action to update your local repo with any changes files. This does NOT affect your branch, but will show you any changes such as new commits etc. You can configure it to prune (remove) any "stale" branches.
Push - An action to send your commits up to the remote repo. This does not need to be done every commit.
Pull - An action to retrieve new commits from the remote repo and put them into your local repo, updating your version of the branch to match the remote repo.
Rebase - This takes the changes done in your local branch, and redoes them on top of the selected commit. Do this to include new changes from the original branch in your modified branch. IE, you are adding quests to feature/WitcheryQuests, and someone pushes a config change (unrelated to quests) to master. Checkout your branch. Right-click on the commit for the master branch, select Rebase, and now your quest changes will be on top of the new config change. Ideally there should be no conflicts. If there is a conflict, flag staff.
Merge - This has to be done when two people are editing the same file. Ideally this should be avoided.
Pull Request (PR) - Once you have completed your changes, publish your branch to your forked repo. On github, you can goto your forked repo page and do a Pull Request to send the branch to the source repo owners and ask permission for them to add the changes to a branch of their repo, e.g. upstream/master.

Working on Quests for GTNH with GitHub Desktop

Fork the GT-New-Horizons-Modpack repo to your github account. This is not actually needed if Dream has added you as a developer.
Add the repo to your GitHub Desktop by clicking Code and then Open with GitHub Desktop. When asked, choose contribute to parent repo. That way new branches are automatically based on the GTNewHorizons/GT-New-Horizons-Modpack master branch.
Before you begin making changes, you want to click Fetch origin to get the latest updates and then start a new branch (Branch, then New Branch). If you followed 2. that will automatically be based on the GTNewHorizons/GT-New-Horizons-Modpack master branch.
Start GTNH. You need the full game (the latest release or a nightly version), not a development environment. You should also check that you are using the latest BQ from https://github.com/GTNewHorizons/BetterQuesting/releases (you should only use normal versions, not pre-versions).
Replace the DefaultQuests folder in your config/betterquesting game files with the one from your github folder. That means to delete the existing folder and then copy in the one from github, do not merge them!
Run /bq_admin default load in the game chat. (Apparently it can crash if you run it immediately when entering a world, so wait a few seconds to be sure.)
Double check your questbook is indeed up to date with the latest changes on GTNewHorizons/GT-New-Horizons-Modpack.
Use /bq_admin edit to unlock edit mode. Now you can actually start editing quests!
While doing changes, make regular commits to your branch. To do so start by using /bq_admin default save. LOOK REALLY CLOSELY AT THE COMMAND AND MAKE SURE IT SAYS SAVE. LIKE SERIOUSLY STARE AT IT.
Then replace the github DefaultQuests folder with the one from your game. That means to delete the existing folder and then copy in the one from your game, do not merge them!
Then write a mini-summary and press commit in GitHub Desktop.
Ideally do one quest per commit. Doing regular commits makes it easy to review your changes or to revert specific things! It also backs up your work.
Once finished, compare the resulting changes and make sure everything is as expected.
Then publish your branch and make a PR to the GTNewHorizons/GT-New-Horizons-Modpack master branch. For example you can go to your forked repo on github and make sure your branch has shown up there. An option should show up there to "create pull request".
List all your changes in the description of the PR. And add screenshots where useful.
Write 'fixes #issue' in the PR description to automatically link a ticket to the PR.
If changes to your PR are required, you can just make more commits on your branch and push them. Make sure to comment on the changes in the description.
After the PR is merged, you can delete your branch.

New Quests

Open the quest book and click Edit at the bottom.
Click the tab you want to put the quest in, then click Designer.
Click New Quest, and place it in the quest graph.
Click Select Icon and choose an appropriate icon for the quest.
To define requirements, select Edit, then Requirements, then search for the requirement quest on the right hand side with the quest ID or the title. Select + to add the quest to the requirements. The eye symbol lets you select if the connection is always shown/always hidden/only shown with hovering over or shift. Ideally you want to show dependencies but avoid always shown if it leads to excessive line crossing. You can right-click a quest and click "Copy Quest ID" to copy it to your clipboard.
Select Open and open the quest for editing.
Select Edit, then you will see the Quest Name and Description. When editing the quest Description text press "Aa" to get a large window. Hit Enter two times once you have a few lines of data. This breaks up the text and makes it easier to read. Keep in mind that quest names in the GT Tier tabs are color coded.
Form the Edit Quest window, select Tasks to add new tasks.
For Tasks we usually use several optional retrieval and one retrieval. Other options are possible too but should only be used for a good reason.
There are many more things to fine-tune: you can toggle if NBT data should matter, can delete parts of the NBT data of the required item, can allow for oredict substitutes, etc. For example, in a bee quest you should delete all NBT data except for the species and then make sure ignoreNBT is set to false.
Select Rewards to add rewards. You can add Choice where the user chooses between items on a list, or item where they get all the items. Generally try to reward similar amounts as other quests in that tab.
In Edit > Advanced, you can change additional properties. For example you can reorder the tasks, change the quest cooldown if it should have one, or toggle if the quest is a main quest. Be careful if you want to swap tasks around to not accidentally assign the same index to two tasks. That can currently crash the client.

Moving Quests

If you need to move a quest to a new tab, get the quest ID or name. You can right-click a quest and click "Copy Quest ID" to copy it to your clipboard.
Click the destination tab, and select the Add/Remove quests button.
Search for the quest id or name on the right side, click + to add it to the quests for that tab.
Go back to the original tab and select the Add/Remove quests button.
Find the quest in the left side, and press "-" to remove it.
In most cases, the quest should remain in one tab to avoid bloat and because dependency lines don't work well for a quest that is in multiple tabs at the same time.
However, the quest can be in multiple tabs if necessary. Quests with important info should be added to the Tips and Tricks tab. Don't remove them from their original location in that case.

Deleting Quests

Quest IDs are now randomly-generated UUIDs, so we are free to delete quests where appropriate.

Testing Quests

To test quests you need to use /bq_admin edit to leave editing mode.
Make sure to craft items and not just take them from NEI. Sometimes the ones in NEI are not quite correct (metadata, NBT, etc.).
You might have to complete the prerequisites to unlock a quest and test it. You can quickly do so in the designer mode. Once the prerequisites are unlocked, you can use /bq_admin edit to leave editing mode.
Do a /bq_admin reset all to reset your quests back.

Troubleshooting

BQ is replacing quest items that I did not touch with betterquesting:placeholder: That means these items do not exist in your game. Maybe they are new and you are not using a nightly version of the pack. In that case you should update. Alternatively, maybe you removed a mod or changed some mod configurations. In either case, do not commit such changes.

Translating the Questbook

Whenever the questbook data is updated in GitHub, our automated workflow will update config/txloader/load/betterquesting/lang/template.lang, which is a template for translating the questbook. By itself, this file does nothing, since it just contains the default English text for the quests.
NOTE: template.lang will be regenerated automatically, so you shouldn't manually modify it, nor include it in your PRs.
Translators can work off of the translation keys in template.lang to create a translated lang file. Such files will go in the same directory; e.g., config/txloader/load/betterquesting/lang/zh_CN.lang for Chinese translations.
If you're modifying quests and want an updated template.lang without going to the trouble of pushing to GitHub, you can run /bq_admin default exportlang to generate a template.lang based on your current quest data. This file can be found in config/betterquesting/DefaultQuests/template.lang. Do NOT commit this file to GitHub!
The command in (4) also makes it possible to perform the translation directly in the questbook in-game. Write the translated text directly into the questbook, export template.lang, and then extract the translated entries manually. The diff command would probably be helpful here, diffing against the original template.lang.