Fan fictions are a bit different than actual novels. In an original book I recommend planning out everything to a T. Fan fictions though, the main thing you have to focus on is what is the main goal, and what is the conflict. I've stopped reading books because it was happy and peaceful all the time-there weren't any struggles beyond a three paragraph argument. If you have a bad guy, ask yourself what he wants to accomplish. Ask yourself how you want it to summarized up. If you don't know where you're heading, you can easily get steered down the wrong path. You see this in stories especially when they have a hundred different conflicts. One moment they'll be fighting a monkey, then the monkey will be defeated and all the sudden they're sucked into the future by a hundred years. Next thing after that they're fighting a gang of mobsters. You don't need to plan it out, but you do need a few bulletin points:)
