Step 9: Make the Gusset and Sew the Bottom
Turn the bag inside out so the lining fabric is on the outside. To make the gussets, fold the sides of the bag over towards the center as illustrated. Pin the bottom so it won't slip when you sew. It might seem counterintuitive to fold the bag this way but when you right the bag and push the corners out, you'll have perfect gussets. Be careful to fold so that the handles are folded just shy of the halfway point. If you make the folds too large, the inside corner of your bag will show out of the top. Not a tragedy but goofy-looking and irritating if you're a perfectionist like I am.
I used a finishing stitch across the bottom to make it very secure. You don't want your onions or canned beans falling out onto your toe. If you've got a serger, you can use that or you can zig-zag. Whatever you do, be sure and use a stitch you're sure is going to hold.