Because t-shirt fabric is stretchy, it is really easy to make ruffles. You need to stretch the fabric out at the sides while pushing forward at the same time, This creates the perfect t-shirt fabric ruffles without having to mess around pulling threads.
If you don't have a ruffler: The easiest way to make a ruffle! highest tension + longest stitch = automatic ruffle - Going to have to try this! I have a ruffler foot but I hate it and I can't figure it out to save my life!
Tutorial for easy DIY knot headbands. Cute, simple look. There's also a quick tutorial near the bottom of the post about making one of these out of a non-stretchy fabric, sewing part of headband to be more elastic-like.