First up, stitch together two squares of any woven fabric with a straight stitch on your sewing machine. To make crisp corners, leave the needle down while you lift the presser foot and pivot the fabric around the needle.
Stitch most of the way around, but leave an opening on one side at least three fingers wide. Clip the corners at a 45 degree angle and turn the bag right side out. Use a pencil or chopstick to push out the corners, then press with an iron.