I work out scenes the same way I work out plots, I have key points in mind and work toward them, but sometimes one thing leads to another and things go in a totally different direction. If it gets to that point, I either try to steer course back to the original idea or see where things go from there.
My most recent work, pretty much organically. Establish the characters, setting, and prompting circumstances and start describing the action. This describes my contributions to Slut World and to Lusty Magical Academy, where the PBTA game format prompts the direction that a scene ends up going. When The Cat's Away has some more specifically planned scenes, particularly the party / orgy / group ones. Similarly, Lucy's Portion and Office Morale are a little more structured in order to fit the scenes into the storyline. My contributions to At the Cabin, because they're framed around a confined time and space with multiple characters, have sex scenes that need more planning in order to fit into the established circumstances.
I may have one or two actual sex acts in mind for a scene, mainly if I'm working through a developing physical relationship and don't want to throw the kitchen sink in on date #3. But beyond that it's just what the characters seem to be into, but I do try to keep some dialogue in the sex as it helps me think about what the characters would be thinking and desiring in the moment and that also influences the action.
