But how will you get your fix of "Average Alex suddenly from his unhappy life into best friends in Ponyville"?>>7284
Porn is about lust, right? If you understand each character's lust (how they feel and why) and can communicate it, I feel like you're most of the way there. The mechanical aspects aren't really paramount.
If you need a premise, /anon/ is always happy to have people write porn to their specifications.