Here is how I have done it (to fit age 3-6 yrs approx):
Cut out 2 strips of fabric, I have used scrap jersey material so they have a bit of stretch:
Headband strip is 35cm x 9cm
Smaller strip, for the elastic casing, is 24cm x 5.5cm. (forgot to take a pic unfolded!)
Cut one, 12cm, strip of elastic 2cm wide elastic.
Fold the two strips in half length ways, right sides together, and stitch (I used a 0.5cm seam allowance).
Turn right sides out then press seams down the middle and you will end up with:
Attach a safety pin to the elastic and thread it through the smaller tube until it is just at the end.
The safety pin will still be in the tube at this point but first the most important thing is to anchor the elastic at the end of the tube. I had to unpick the first headband twice so decided this step was essential!! I pulled the top of the casing away from the elastic and stitched throught the elastic and one layer of material.
Then fold over the end of the tube ~0.5cm,
gather one end of the larger tube (the pic shows a gathering stitch but in the end I just gathered them by hand),
Stuff the gathered headband piece in to the elastic tube (make sure the seams are both on the underside) then stitch in place. I stitched twice to make sure it was really strong.
Now, pull the elastic to the other end of the tube and repeat the above until you have something like this:
I made the flowers using this tutorial by Little Miss Momma, hopefully you won't burn yourself with the hot glue!!
I have made 8 of these and hopefully the girls at the party will like them!!