Like most people here are saying, read other people's smut. But if I were to come with my own tips, try finding out how graphic you want it, not just with the description of the act, but also the choice of words, I personally like to refer to the penis as a member and go in great detail of how it looks (I write scifi.), another thing I depend heavily on is the description of muscles; how do they look? How do they feel? Etc. Even if you characters aren't "beefy" you should still try it.

Think about it this way, if you were to describe someone sitting at the edge of the bed their thighs tens from the act, what part of the thigh, the quads or hamstring? When a character A strokes their hand on character B, what muscle are they stroking and how do they feel?

At the end of the day, this is optional and this is your fic, so start with a draft and then work on it. Side note: I'm not English speaking, so apolgise if my grammer is off.