You first need to spread the towel wide and then pick one long side and one long side. Once you are done with picking one from both, use a ribbon and sew them both. Now you need to turn the towel the opposite way so that you can see the ribbon right at the bottom of the towel.
After you are done turning the towel the opposite way, you have to fold the towel from the long side and then pin it. Once the fold has been made, sew it but not from the edge. In fact, leave a little space between the edge and the area you are trying to sew.
Grab elastic and wrap it around your waist. When you know how much will fit around your waist, cut it from there and place it on top of the towel for a while. Make sure that the elastic that you have cut will not be too tight around your waist.
Now you have to sew the elastic on the towel and at both the ends so that you can use the elastic when you are wrapping the towel around your waist. Make sure that you sew the elastic on the side where the ribbon side is on the outer edge and you can see the ribbon and not the elastic when you are wearing the towel around your waist.
You will be sewing the elastic on the opposite side of the towel, so you will basically be holding the towel from the wrong side. Once you are done with the sewing bit, turn the towel so that the ribbon is revealed. Now it is time for you to sew the opposite end of the towel and the ribbon. You are good to go. Try to wrap the towel around your waist and try to see whether it slips down or not. If it does, use smaller elastic; if it is too tight, then replace it with long elastic.