I seen it done but never done it. I know that you need a good stabilizer, cut the fronts or backs (wherever the logos are) out a lot bigger than what you want. Iron on fusible stabilizer, then trim them down to the size you want. Since tshirts are heavier than regular cotton and you have the added wieght of the stabilizer, 20 is the max that you should go in one quilt. More than that will make it too heavy. After you get the stabilizer ironed on, the blocks cut to size then it's just a matter of how you want to put it together. The prettiest ones I've seen have used cornerstones and sashing. Then just sandwich and quilt. Your location says in hiding but if you're anywhere near Danville, Il., go to Threads of Time on a Monday or Tuesday, they have a T-shirt quilt expert that works there on those days. Call first, she's leaving on vacation soon.
http://www.threadsoftimefab.com