lightweight fusable non-woven stablizer- available at stores like joannes-
yes there is an actual t=shirt pattern on the market-
it is called "t-shirt memory quilt from Four Corners Designs--designed by Marilyn Robinson.
Use the lightest weight fusible stablizer you can find. Making sure that you put the stretch of the stablizer crosswise of the stretch of the t shirt. I cut the stabilizer into pieces slightly larger than I want, fuse it to the shirt and then cut the block to size.
Use a wee touch of a washable glue stick to help hold your seams open rather than pressing to one side (less bulk). Press nicely but remember not to press over any rubberized, painted, etc. emblems on the t shirt.