Once there was a store in my hometown that sold hairbands covered with ugly, dry.. well, it looked like gras. Really! And it was ugly, BUT the hairband had a base! Plus, by reasons of their crappy cheap outlooks, nobody wanted to buy them and they were 50 Cent one. So I bought several of them and removed the 'gras'...
It is important to have a pattern for the hairband you want to alter. The left one is for those hairbands which are broad in the middle, like for example the polka dot one in the main picture.
Measure them! And double the size breadthwise so the fabric can be folded around the whole hairband.
You can either glue it on the inside or handsew it. Since I have a hot glue gun (thank god for them), I did it that way, because it saves time.
Then you can decorate everything if you want to :)