First of all, I'd like to state that this takes minimal sewing skills. If you can sew a straight line, you can make one. Fear not! It also doesn't take much fabric, so it's a good way to use up scraps.
The best way I've found to do this is to make a pattern with freezer paper first instead of trying to measure & cut out on the fabric its self.
You need one strip of fabric measuring 7 inches wide by 16 inches long (well call this one A), a second 3 inches wide & 12 inches long (and this one B), and a 6 inch strip of elastic between 2/3 of an inch and an inch wide.
On strip A fold the long sides of the fabric in 1/4 inch, then 1/4 inch again on the wrong side of the fabric so the raw edges are completely hidden. On the short ends, you only need to fold them in once as this end will be hidden. Press down & stitch down all the way around the strip. You can either use a straight stitch in a color thread that will blend in, or a contrasting thread with a zig-zag stitch. I used the zig-zag contrasting stitch for the more casual head bands.
Next, fold the short ends of Strip B down to the wrong side of the fabric, press, and stitch. Then fold the band lengthwise with right sides together, and seam along the length only (5/8 inch seam allowance). This will be the casing for the elastic. Turn right side out & press.
Take Strip A & pleat the ends together (it doesn't have to be a perfect pleat) until the end is small enough to fit into the casing for the elastic. I whip stitch the pleats closed by hand. Then secure one side of the elastic band to one end of Strip A. Thread the elastic through the casing & sew the opposite end of the elastic to the opposite end of Strip A.