i was gonna comment this on your oneshot, but the yapping just kept going, so i’ll put it here instead:
i love love love this so much! it’s not even as bad as you think it is. <3 as for the advice: mainly, write what you are comfortable with. smut ranges from very vanilla, to very, very explicit. make sure you have a clear idea of the characters positions, and have fun with it. without worrying how good or bad it is, just start writing. it can feel a bit awkward at first, but it definitely gets easier when you think of it as just another scene. i think it's important to focus less on the specific acts and more on what the characters are feeling emotionally and physically.
2: read lots of good smut. think about what makes it good (is is interesting, emotional, arousing, hot?) you can also read smut that doesn't land quite right for you and think about why that is too. before i started writing, i just took bits and parts of what i liked from other writers and used it in my own works. brainstorming and research is very helpful! i like to use a smut thesaurus from google/tumblr to help with word choices.
also, porn can be helpful when you’re using kinks and things like that.