Use a wide zigzag stitch to secure the ends of the zipper and the mats. Be careful not to stitch into the zipper thereby breaking your needle! Trust me, I speak with experience.
Open the zipper to facilitate turning the bag. With right sides together sew around the remaining three sides of the bag using approximately 1/4 inch seam allowance.
Open seam at the bottom of the bag and match seam line of bottom with side as shown forming triangle. Stitch across the bottom of the triangle. This will form the base of the bag. Turn the bag to the right side.