Fusible Interfacing -- Not Fusible webbing ( like wonder under) you need interfacing which is fusible on one side- and keeps the t- shirts from stretching. It is easiest to fuse the interfacing to to t-shirts before cutting them out. There are many free videos on t-shirt quilts, along with Pinterest , google images and the photo section here on the board to see different layouts & ideas. Sometimes they are cut all different sizes, filled in with smashing strips and other blocks, sometimes all one size. Do a little browsing, and looking at the shirts you will be using to choose a design you like. T- shirt quilts can be quite fun. Use a Lightweight interfacing- t-shirts make heavy quilts.