Take your original headband, something that you don't really wear anymore. Too bland perhaps?
Just add some scrap fabric and it's as good as new!

Anyways, to be sure of how much fabric you'll need, measure how long the band is. You don't necessarily need to do this because it's fairly easy to guess or just lay out on the fabric.
This headband was about 15 inches.

Next, measure along the fabric that you want to use. I used a straight-edged ruler too since measuring tape is kind of annoying on fabric.
Measured out 15 inches along the edge of the fabric.
It might be better to give more space height wise since when I was putting it together, it got a little close near the middle.

So the fabric is all cut! I did mine about 1.5 inches but the more the merrier, and any excess can be trimmed.

And now the gluing. :)
Place hot glue along the edges of the inside of the headband, and fold over the strip of fabric. Just keep going until you wrap up the whole thing.

And it's finished. :D