Sew a 1/4 inch seam all the way around the bag except for a small 3 inch opening on one side just below the curve of the top for turning the bag right side out. It is a bit of a tricky turn but it can be done!
The bag will turn inside out a bit and needs to be turned so the outer pocket is on the outside and the ribbons are on the outer edges of the bag. This picture shows the bag still needs that final turn of the bottom piece.
You will need to fiddle with it a bit to get the corners flat and square. Then give it a good iron.