This is probably one of the simplest sewing projects ever, but I made a few miserable prototypes before arriving at it. So to spare you the same frustration: a tutorial!
Most hairband patterns I've found involve sewing two pieces of fabric together. However hard I try, I always end up with something wobbly, like this:
Topstitching only seems to make it worse, so I'm not even going to show you that. I'm sure with patience it could be done neatly, but how long should a hairband take to make, really?
Another delightful thing about this hairband is that it won't mess up your bangs when you put it on, because it can be fastened at the back rather than being pushed back onto your head.
First, measure the length of the band.
Draw a line for the length, and mark the middle. Decide how wide the band should be at the ends and the top. (Don't worry about seam allowances yet.) Mark the widths and join the points up.
Cut this pattern out, then trace around it onto another piece of paper. Now cut the first pattern along its mid-lines. Stick it to the edges of the new pattern, placing the long mid-line edges at the outside. Add a seam allowance all the way round, smoothing the points at the widest part.
The outer edges are now longer than the middle of the pattern piece; to compensate for the wedge-shaped pieces that have been added to the widest part, take off something similar at the ends. (This doesn't have to perfect - just guess.)
Cut the pattern out of fabric. Start heating your iron up. Fold the fabric in half and sew the seam, leaving a space to turn it inside out. Leaving the gap closer to the middle makes it easier to turn.
Press the seam open so that it lies along the middle of the band. Sew the ends shut, clip the corners, and turn it inside out. Close the turning gap by hand.
Sew buttons on the ends (on the right side).
Loop an elastic band around the buttons to fasten.