Once again, this is a separate topic that requires its own guide, but here is a really helpful post by a Reddit user. The short version of it, though, is that it takes more than a little bit of creativity and imagination.  Not only do you need to have a general idea for a plot, but you also need to know about the different side quests and plots you want to throw in. Of course, the biggest rule is to have fun while doing so, and keep an open mind, as pretty much anything can happen during a game!