Things you will need:

Step 1: Measurements

Measure around your head (over your ears), I am about 24", I gave myself some stitching room.
Then think about how wide you want your head band, I wanted about 3.5" so I went 4" to give myself sewing room.
Outline the rectangle, try to make it as straight as possible (the straight edge would be handy here)
Make a second rectangle (I cut one out and traced it.)
Cut the rectangles out!

Step 2: Pin It, Stitch It.

Make sure the pieces are lined up on top one another.
Sew down each of the long sides.
Turn it inside out
You can hand sew the inside or try to sew it with a sewing machine (or what ever technique you know) I Wiggled the piece under the machine and sewed as much as I could and hand stitched the rest.
The headband itself is done!

I make a flower to go on my head band but there are SO MANY different things you can do!

Make a cut out that looks like a flower petal ( I just go creative)
Trace anywhere from 3 petals and up (for the headband i actually did 6 petals, you can do more to make it look more full)
Knot the end of the thread well then make a running stitch through each petal and pull them together so that they ruffle.
to make the center bud just cut out a small circle about the size of a quarter and do a running stitch all the way around it and pull it so it makes a puff ball.
stitch the bud into the center of the flower then onto the hat! done!