there are tons of t-shirt quilt patterns on the market and many tutorials - and u-tube videos---just do a search- but
easy explanation-
cut out decals/designs for shirts, press a lightweight fusable stablizer/interfacing to the back of each one- lay them out the way you like them- add sashings/blocks to create a quilt top- sew it all together- quilt & bind. the key is the fusable interfacing (which controls the stretch)