I have made probably 20 t-shirt quilts. I use pellon P44F fusible interfacing which is 17” wide. I usually cut it larger then the square size I want. I add interfacing before I cut to size that keeps your blocks from stretching while you cut.
do be careful and don’t get your iron too hot or you could melt the design and the interfacing. The P44F is inexpensive and since it it lightweight it does not add bulk and adheres with a lower iron temp.
now I usually but a cotton banner between the blocks, that helps stabilize things and eliminates some of the stretch. If you stitch tshirt to tshirt you have more stretch to deal with. I usually quilt around the design as mine are all custom hand guided and just do a tight meander on the open space. I try not to stitch into the shirt paint as you will get skipped stitches and it will dull your needle.
I also use a poly batting as t-shirts can be heavy. And a cotton batting is more weighty. Also you will find that not all designs are on the shirts straight. I usually line up my ruler on the design and cut on an angle if I have to so it will look straight on the quilt.
here is a pic of one of mine.
