my two cents would be:
1. go with what turns you on; seriously, your internal porn cinema is evolutional the best mother nature could develope - just use it
2. rise the tension until the final climax; means structure in waves, up down, vary the speed
3. don´t be afraid to look how others write if you need an idea how to write; this is the internet! just check how other authors build sentences
4. in my opinion detailed describings help a lot;
5. avoid repetitions!; trully, to read: he fucked her ... he fucked her ... than he fucked her ... finally he fucked her hard ... is kind of turn off to read.
if it isn´t a spoiler, what kind of smut? BL, GL, Straight, or ...?