I haven't made one myself but have helped with several. Cut out the portion of the tee you want to use. Cut it larger than the finished block size. Cut a piece of knit fusible at approximately the finished block size. Fuse the two together with the tee knit stretching from east and west and the fusible stretching north and south. This will stabilize the stretch. Make sure they are securely fused, cooled. Then cut to the finished size and use as you would any other block.
I think it is important to sash your blocks as this will help stabilize the block. Note, the blocks do not need to be the same size but will require some planning.
These t-shirt quilts are great for teens and a wonderful quilt for a student to take to college. It will be a one-of-a-kind and less likely to take a walk if you know what I mean.
Meanwhile, you young mothers start gathering significant t-shirts for use later. Suzanne