What you need:
Take wrong sides of fabric and match up the zipper then sew together, repeat on other side.
Sew across zipper creating a seam. Then match up wrong side of small rectangle to seam and sew on.
Match up wrong sides of side panel and sew down creating box shape for bag. Repeat on other side.
Sew wrong sides of bottom seams together. Take side edges and sew flat to create box bottom for bag.
Flip right side out and voila!