Any tips for a beginner?

Hi! So as you can probably tell, I’ve never written a proper sexy story before, but I think I’d like to try it. I’ve been doing role-plays on r/dirtypenpals for almost two years and enjoy the writing and creativity aspect of it, but it’s also in relatively short form and the partner dictates half of it. I’d like to take a shot at writing my own sex story but I have no real writing or storytelling experience. So from anyone that knows what they’re doing, is there a process or do I just wing it? Thanks in advance :)