i never did oneshots, launched straight into the great longform beast. i kinda regret it, but also dont - its what i needed at the time, its what i continue to need to keep me working, but it means that im left with a lot of decisions i wouldnt have made now, and im playing catchup with myself.
however, im working on making oneshots alongside. the most important piece of advice i can give is spend some time with your characters - its through your characters that your audience see your world, and you want them to resonate. you can go through this methodically, jotting down lists of traits and attributes, but really, i just recommend you write some dialogue between your characters for fun, and just play with them in your head. think about how they mesh together, what they want and what they lack - if you cant find a plot, build it from your characters needs.
also, a really good take from filmmaker taika waititi on short stories:
"Short film: you can be poetic and you don't have to answer anything. You can make whatever you want. You have creative freedom with short film."
dont be afraid to leave things untied, and really have fun with it. these are your exercises, so look for the things you really want to see done - make a wish list of every story you want to read, every line or character or way of telling a story that you think would be really cool. draw from that.