Start by measuring the circumference of your head. Mine's 20 inches; so I estimated that 16 inches would get me from behind one ear to behind the other, joined by a piece of elastic outta keep the band on snugly and attractively. Then I cut these rectangles that were roughly 16" x 4".
Next step, iron right sides together lengthwise and sew raw edges together with approx 1/4" seam allowance. I like to use pinking shears to trim the seams allowances and prevent fraying, but it's probably not necessary given that these seams will be inside the finished band.
Pull one end of the tube through the other to make them right-side out. My preferred method is pinning a safety pin on one end then working it through the tube.
Iron them again, folding the raw edges inside about 1/2" at each end.
The idea on these hairbands is not mine, and, if you're reading this blog and remember that you actually gave me this idea, I will insert your name here, Friend. And maybe give you one of these babies.