I use a non-woven iron on interfacing to stabilize, usually something on sale at Joann Fabrics. I figure about a half a yard per shirt, but I usually just buy the bolt. Then I cut the shirts at the shoulders, armholes to remove the sleeves and then down the sides. I fuse the stabilizer to the wrong side of the tee shirt. I lay a Teflon pressing or applique sheet on the ironing board first and place the front side of the tee shirt on it and the interfacing, glue side down onto the wrong side of the tee shirt and press. Let it cool before lifting the tee shirt off. The Teflon sheet helps prevent the design from coming off the tee shirt. Then and only then, cut the tee shirt into the size block you want it to be. Then you can sew it like any other fabric.