You need worsted cotton yarn and g hook, as well at a large button.
First you need to measure the head where the headband will sit. I measured my two year old's head to be 19".
Then you double that length. ( 38") and crochet a chain for just about a half inch shorter than your doubled length. (So I crocheted a chain that was 37 1/2 " long)
Now you need a big or cute button. I like the ones with a loop on the back.
Thread one end through the loop going one direction.
Then thread the other end of the chain through the loop going the other direction. Tie a really tight knot, then a sailor's knot. Clip the ends short.
Flip it over and you are finished!
Just wrap it around your head and loop the chain over the button! Easy peasy!!