After you've stitched the three sides of your bag, you're going
to box the corners so the bag sits a little wider.
At the bottom corners of the bag, pinch the side and bottom seams
together and flatten them. Then measure up about 3.5 inches. Mark a
line across the corner at this height and pin the flattened corner in place.
Do this on both sides, making sure you're measuring up to the
same height on each side.