At the top of the front panel, measure three inches down from the middle crease mark. Mark it with your chalk. Now measure one inch on either side. Mark these points, too. This is where your drawstring holes will be.
Use a grommet punch to make drawstring holes the fast and easy way. It takes seconds, while cutting and sewing button holes can take up to an hour. The grommets also hold up well under stress and repeated washings.
Otherwise, you will need to sew button holes. Carefully cut two vertical half inch slits at your chalk marks. Use a needle & thread to sew around the slits to reinforce the holes. You will need to cover the holes. A sewing machine can also be used, set on 'button hole' mode.
Now you can stitch the panels together.