To become an editor, type /ftbquests editing_mode command (You don't have to have cheats enabled in singleplayer worlds for this command to work). New buttons will show up in the Quest GUI.

Click on + button on left to add a new chapter.

Right-click anywhere in chapter page to add a new quest.

Right-click on any chapter, quest, task or reward to open context menu from where you can edit objects, delete, move, etc.

To make quest A depend on quest B, select quest B and Right-click on quest A and click Add selected quest as dependency. To remove it, click Remove selected quest from dependencies instead.

Most objects have Reset Progress and Complete Instantly available for testing purposes, it only affects your team.