It's also a great size for an overnight bag, a reusable grocery bag, etc.
Step 1: Cut bag shape
If you have pockets, use those as a guide for cutting each square. I folded my pockets in half then cut around them. My panels ended up being 16" tall and about 13" wide. I also used some scrap fabric to make a liner the same size.
Step 2: Cut straps
Step 3: Sew liner and straps to bag
Then lay each panel out flat and line up the handle. Start at the bottom and then pin up the front against the pocket. Stop pinning when you reach the seam of the liner.
Make sure the strap isn't twisted and pin it back down the other side of the pocket. The raw edge of the straps should line up with the bottom of the panel on both sides of the pocket.
Start at the bottom of one side of the handle and sew a seam close to the edge all the way up until you reach the seam of the liner, turn and sew across strap, turn again and sew back down. Repeat 3 more times for each side of both straps.