Cut a rectangle of oil cloth measuring 16" by 20".
Pin your oil cloth face down on to your zip along the top 16" edge.
Use a zipper foot to sew the zip in place.
Fold over, a pin the other side of the zip to the bottom edge of the oil cloth. Sew in place.
Cut another small rectangle of oil cloth, fold in half and sew along both edges to make a loop.
Position the zip, so that it's in the center of the bag. Fold the loop over, and place inside the bag, so that the open edges are sticking out in line with the zip and sew along the open edges of the bag.
Cut 2" rectangles out of each corner of the bag.
Squish the edges together at the corners and pin in place.