It's done from the offensive stance and depends upon positioning and arousal level. A kneeling opponent can be grappled into performing oral if you're standing and have enough arousal, for instance. Wrestling is the standard type of grapple, performed from the offensive (yellow) stance, and once you're engaged the goal is to get into the dominant position. You'll see an indicator; if it moves toward the left it means you're losing while moving toward the right means that you're winning. While you're struggling you can force the opponent into a number of positions.
Not every opponent can be grappled. Some are too big or airborne.
