While holding the tape measure at 22" grab the fabric in two spots and see if it will stretch to 22". I was going to take pictures of this but I don't have three hands, so pictures of me holding/measuring the fabric are pretty impossible. Here's kinda what you do with a paint picture:
Cut a rectangle out. Mine ended up being 5" x 16". (Keep in mind my knit was stretchier than a tshirt, so if you're using a tshirt you might need it to be longer)
(just ignore my leg in the picture, I was running into the whole not having three hands problem again)Put it on your head (or somebody else's head).
And you're done. No need to sew up the edges because knit fabric won't fray. I see more knit headbands in my future! What's your solution for bad hair days?