You need a piece of fabric 1.5″ wide (for 1/2 wide headband) by about 18″ long. You can also use ribbon- which would be waaayyycute!
Lay fabric piece over headband. Pin behind ear on one side. Pull taught and pin behind ear on other side. Make sure the fabric is taught, as it will stretch a bit and if it is too big it won’t work. Take off your head.