If I have the basic idea, I always start with drawing out characters and gestures. I like to get a feel for the mood I'm trying to go with, and usually for me, quick 1-2 min gestures help. I get character expressions, interactions, conflict, and then I start to form a simple plot from that.
Music also helps with this process, especially when I'm writing the script. Once I have characters and conflict in mind, I create playlists that I think helps with the mood I'm trying to convey. For instance, if I want to write a horror story that has kinda of a hippie vibe to it, I get songs that were made around the 60s, 70s, even modern songs with a psychedelic feel to them.