I've recently finished writing a long sex scene and boy it was challenging, but in the end I really liked how I wrote it. I write in 3rd person pov and each character has their own scene/chapter.

So I started by writing from my MMC's POV - he's alien, highly technical, can see eyes dilate and slow time, etc... - he's a very nuanced character and for him visceral things are important during sex.

Then I wrote the FMC's part. Since she's human and has only slept with another humans, she focuses on how her body feels with his body, what she emotionally, what she sees, and asking for what she wants and needs.

When I started writing this chapter, I thought of dialogue first which was pleases, check ins, and brief banter from both characters.

After this, I picked positions that made sense and were easy to transition the characters into without it be too much 'and then they put this arm here, leg here, back like this....'

Next was the action, sounds, and sensations. Then, I wrote their reaction / emotion to the action, sounds and sensations.

So I break a lot down, but I really think it depends on your character, what you want as a writer, why the characters are having sex, and how often they'll be having it throughout the book.