@Fixed20 So, for me, it's about picking a moment or event from the original story that helps me. Like a scene that you really liked or found interesting. Then, focus on expanding upon that moment, then exploring it from different angles. Like, per se, Obi-Wan gets a new Padawan during the Clone Wars (or whatever you have in mind). You can add your own spin on it from there, and/or add new characters along the way. But as long as you stay true to the characters, your OCs, themes, and overall feel, it should make for a compelling fan fiction.
P.S. sorry this was a little long, but I actually had to think about this a bit, being somewhat new to writing fanfic myself. I hope it helped!