This tutorial was fun and super quick. I've had a couple of shirts that I got as gifts a few years ago and have never worn them, so instead of throwing them away, I'm refashioning them.
For this one, you need 2 strips that are 24" long and 2.5" wide. I folded my fabric in half so I only had to cut 12" strips. It made it a little bit easier to cut since I've got a small table.
Using a rounded object (I used a gigantic button) make each end of your fabric equally rounded
Pin your fabric together with the wrong sides together and sew, leaving a 1" space right in the middle of your headband. Use this space to flip your fabric right side out. Iron it so that your headband is an even width.
Since the gap in your seam lands right at the nape of your neck, you can zigzag over the hole to stitch it closed. If you don't want your stitch to show, feel free to hand stitch it closed.
What I really like about this headband is that you can wear it with pretty much anything.