This project is perfect for pre-cut slices or jelly rolls. The one I'll be using is 2.5 inches by 42 inches. Start by measuring around your head, and subtracting 2 inches - cut this length from your strip of fabric. Now fold this piece lengthwise, right sides together, and sew all along the edge.
Now cut a small piece of elastic about 2 inches long, and push it into one end of the tube you just created. At this point you need to decide where you want your seam to be. For this headband I left the seam on the edge, but you may want to move the seam to the middle so that it is hidden under the headband.
Push the elastic piece in until the edge lines up with the edge of the fabric, and center it.
Now stitch back and forth across the seam a few times to hold the elastic firmly in place.
Next you need to flip the tube inside out. This can be a bit tricky, but if you have a knitting needle handy it's a breeze! Start by pushing the elastic inwards a bit, just enough to get the end of the needle in. Then push the elastic end all the way through the tube with the blunt end of the needle.
If you're adding a bow like I did, you'll want to slide it on now before you connect the ends.
The last step is to tuck the raw edges of the other end in 1/2 of an inch or more (try it on to be sure, you want it to fit snuggly). Then insert the edge of the elastic, and sew across the seam a few times.