Small piece of elastic (mine was 1/2 inch wide but really, most any width will do)
Instructions
Cut out 2 pieces of fabric that are about 3 inches wide by 12 inches wide. (Really you can vary this if you want-depending on how big or small you want your bookmark.)
Cut your elastic about 8 inches long.
Sew your fabric with right sides together. Sew up the two long sides.
Turn it right side out and press it flat.
Fold the ends in and press that as well.
Insert your elastic at the end and sew it in place.