Use a current shopping bag as a guide. Is it too small for you? Would you prefer it boxy or flat? You can use a piece of gift wrap tissue pattern and make a pattern (or newspaper because it is wide). If you have difficulty in figuring out how to box the corners, turn your bag inside out and put a box inside it (a cereal box works well). Push it into the corner and you will see a triangle emerge. This is the line you will want to sew across and match the size on the other side. I do not cut off the corners because I prefer the strength. I also cut a piece of 'granny grate' (plastic needlepoint canvas) to sit in the bottom for stability but easily removed when washing. There are hundreds of free bag patterns available - many on allfreesewing.com and allpeoplequilt.com.