Dyann, I don't know of a pattern for them, but I've made many of them in the past. First cut out the fabric a little larger than what you want showing through the front of the sweatshirt. Turn the sweatshirt inside out and lay the fabric face down on the inside of the shirt. Pin in place. Sew the fabric to the shirt along lines in the fabric you want to cut around. Once all sewing is complete, turn the shirt rightside out. Clip the center of the cutout areas making sure to pull the sweatshirt fabric away from the sewn on fabric so as not to damage it. Once you've clipped each area you can start to clip the rest of the sweatshirt areas close to your stitching.
Have fun!
~Justie~
P.S. If you meant the cut out sweatshirts that have just X's cut out then you sew the fabric in a grid pattern to match the shape you want (heart etc) then you just turn right side out and clip X's in each square. It's a bit tougher than the above, but easily doable.

Sounds like you're talking about "reverse applique".is that correct???
Kathi