I used a light weight fusible woven interfacting. I ironed it on the whole back and front so I would be able to use scraps from one shirt on another to make the blocks as large as I wanted....16 in. I was using shirts from head start through high school, so some of the emblems needed a lots of extra.
I'll post a photo of the t-shirt quilt I did. Been wanting to do that since I joined the board.