Ok, I'm not sure this description will help, but I'll try. You know that hand motion, "come here" that you can do using your index finger, turning your hand palm up and making a C shape with your finger? Think of a woman putting out her hand and signaling: come here big boy.
Well, using that motion, guide your index finger into your ass and start feeling around for a little bump or button inside our butt (it'll be on the other side of where your balls are.) Make sure your nails are cut and you use lube. Feel around until you find that button and play with it. When you're about to come, press on it gently and you'll go nuts.
Sorry if my description sucked, but I've never tried to describe that before typing.