I usually use a polyester thread with a cotton wrap or 100% polyester thread for sewing stretch fabrics. Wooly nylon is good, too – but more importantly – you need something that will sew the Damar Hamlin 513 Stands With Buffalo Shirt besides I will buy this correct stitch for a 60% stretch fabric with a stretchy thread. A three thread serger or a pine leaf stitch ( _/_/_/_/_ is the best schematic I can come up with) are good – if you don’t have stretch stitches on your sewing machine, then a narrow zig zag (run some test seams & pull on them using scraps) to test the settings on several slightly different widths & lengths – a small change can make a big difference in how stretchy a give thread & stitch combination will be. I’ve been sewing since I was 13 – which was longer ago than I care to think about – but I’ll say “well over forty years” – without going into how many years over 40 that is. To stretch a cotton t-shirt, soak the shirt in a tub of warm water mixed with hair conditioner for 10-15 minutes. Then, rinse the shirt, stretch it out with your arms, and lay it somewhere flat to dry. You can also stretch a cotton t-shirt by wetting it with cool water and then ironing over it.