So, this is a tutorial for a regular headband (aka nothing special about it). There are probably a thousand of these out there, and I've used quite a few of them. When I made a headband from other tutorials however, they never ended up fitting exactly how I wanted them to. I'm going to give you the measurements that I used for my adult head, and I'm assuming that they'll work on most adult heads. However, you may want to measure your head or do a trial run with an unimportant piece of fabric just to make sure.
Fabric - 20"x4.5"
Elastic - 4.5"
Step One: Hem
Starting at one of your corners, you're going to want to measure 4" up on the long side (20"side) of your fabric. Mark that spot.
On the short side of your fabric (4.5" side), measure 1" over and mark that spot.
Step Three: Cutting
Get a straight edge of some sort and draw a line that connects the two marks that you just made.
Cut along that line, saving the piece that you cut off.
Step Four: Shaping
Using the piece that you cut off as a guide, cut off all four corners of your fabric, as shown in the picture above.
This is the shape that your rectangle should now resemble. Basically what you're doing is cutting the fabric so that it will comfortably fit behind your ears. I've made headbands from rectangles before without doing all of this, but they don't tend to lay very nicely because they just don't go behind your ears very well.
Step Five: Stitching
Fold the fabric in half long ways placing right sides together and pin in place.
Stitch along the long side of your fabric making sure to leave both of your short ends open. Notice that the stitching in the picture above does not close up the short ends.
Step Six: Top Stitching
Turn your headband right side out and iron flat. At this point the headband should have a boomerang kind of shape to it. Top stitch the long sides once again making sure to leave the short sides open.
Step Seven: Attaching Elastic
Attach the elastic to the headband by placing approximately 1" of it inside one of the openings. While holding the elastic in place, stitch across the opening closing it up with the elastic inside. You'll want to stitch over the elastic at least two or three times to make sure that it doesn't pull out.
Place the other end of the elastic in the other opening and repeat.
Step Eight: Enjoy
Wear your headband with pride!