The Betonquest editor is the easiest way to make long complicated NPC scripts.
Here’s a video series I did on making quests.
Part one: What is a quest?
Part two: Adding Condition
If you want to, you can still edit the npcs the old-fashioned way, opening up the yml files. Even if you do most of the work with the editor, it is a good idea to take a peek into the yml files it creates just to see what it looks like.