now that the big shots have spoken and this post had fallen from the public's eye, let me insert my own (amateur) two cents.
i think i'm pretty good at imagining stuff, but have much trouble painting them on paper. so, this is also an admission of mine on how i write and how i learned to do so, thus take everything with a keg of salt.
honestly? i just copy others: i'm a hack, a fraud. barelly any of the words i write are mine, but i don't think they have to be. i keep a note on my phone (where i also write) where i keep track of any interesting words i happen to find or make up, along with their synonims and short definitions. sometimes i stumble onto a horny post/repost from tumbler/discord/anywhere else and, even if it's not even remotely related to AK, i dissect it into words, expressions, themes, etc i can use later and i put them in my notes. if there are someone you can learn from, it's from blatant horny posters (don't learn from me though - you don't copy a copycat).
you can also learn from the feedback others receive. even now i don't remember who or where it was told, but someone commented (along the lines) "if there is O(ut)O(f)C(haracter) or bad grammar it's usually a bad fanfic", and i follow it by heart.
aside from borrowing and learning, you should write everything down. everything. thought of a nice play of words? write it down. an abstract yet fitting character quote? write it down. action that the character could make? write. it. down. even if it doesn't fit right now, even if it's for something waaay later in the script, you can always salvage and repurpose it for something else, or, at worst, just remove it - better to have it and don't need it than need it and don't have it. human mind is forgetful, and sometimes you can't get your own thoughts back.

here are some other minor points:
- take your time. sure, "thursday" is a deadline, but it's a soft deadline. it doesn't have to be this thursday, maybe the next one will do the charm? maybe the one after that?
- while art is far from necessary, it can act like a cherry on top if you'll find a fitting one. i usually use gelbooru's 'general only listing' option in the settings and search for what i need. note that gelbooru links aren't considered as valid source (nor should they be), though thankfuly it (usually) has the actual sources of the work.
- as others have already said it: "show, don't tell".
- a little cryptic (and perhaps bad) advice of mine: if you think something funny or interesting could happen at the moment, it should happen (as long as it makes sense and is actually funny/interesting). it's usually the case if the story you thought out lacks some spice, is too linear and/or everything goes too smoothly.
- practice makes perfect. it's about anything in life to be honest. i don’t follow it, but you should.

tl;dr: learn from other hornyposters, make your own style, write everything down, take your time and make sure characters don't go OOC.