Here's my little tutorial:
Cut out your pieces from the template
lay side facing each other and sew on each sides
don't sew the ends up
now that you have sew the sides you need to turn it inside out and it will look like this when you are done.
Now tuck in the ends and get the elastic ready to sew in. At this point I also iron it out to make it straighter and easier to handle.
Now you can put the elastic in and sew it up on each sides
Here's what it should look like when you have both sides sewed together
Here are the girls modeling their new headbands. I hope my tutorial wasn't hard to follow. It took me about 5-10 minutes to make both of them.