Step 4: sew all around the edges, except for the place we marked
Using a 1/4 seam allowance, start at the bottom of the opening you marked, and sew all around until you come to the top of the opening.
Make sure to backstitch at the beginning and the end!
Once you're done sewing, remove any remaining pins, including the one inside that was holding the elastic in place.
Trim the edges of the bag, especially the corners! (Do not trim the opening, though.) Cut off the excess elastic cord. (the second photo shows you how it should look - I like to use pinking shears but you don't have to!)