ok so first off you'll see your own junk in the left corner, if it's not erect, you can't do the fucking. You can raise arousal by using seduce, letting them do stuff to you, or sometimes during grapples, they might grind instead of struggle, there is more to it but these are the basics. If you want a quick method, try full Nelson, grapple until you can takedown or judo throw, resume grappling, in the top middle there should be a series of squares, the closer to your side the more advantage they have, the closer to theirs the greater your position is. Getting all the way to their side will allow you to penetrate if you are aroused enough. If you decide to pull out or stand up while aroused, you can fuck them in several stances, one when they are prone/laying down, you can pounce, or facefuck them. If they are kneeling or on all fours you can pounce, or open up (iirc that's what it's called) then if you simply keep grappling/holding while standing and don't take them down you can do a standing penetration when you reach maximum advantage. If you need more tips, try reading the tutorials when you start a new game and/or when you get into your first match.