I just finished one up today. I researched first and found different methods. I chose to use fusible tricot interfacing. I applied it with the stretch going in the opposite direction of the stretch in the t-shirt. It made for a very soft piece to work with that did not stretch! I made the section bigger than I wanted and cut down after I fused the tricot. I think the tricot is a little more expensive than the regular fusible interfacing, but I don't mind...I'm so pleased with the way it came out.
The t-shirts were all different sizes, so I used regular quilting fabric for block borders, then added sashing and borders to the quilt itself. It does weigh more than a traditional quilt. I was going to to FMQ in the area all around the designs on the t-shirts, but found it difficult to do on my machine. I ended up just going around the characters and stitch in the ditch on the block. I love how it came out. I didn't have any problem with puckering. It's in the dryer now, so I will take pics later and add them to this post.