Zip the zipper all the way up so it is closed.
Hold the end without the zipper pull and place it on top of a credit card so you can measure how wide you want your purse to be. Stretch the zipper across the card and mark the spot where it ends with your fingers.
Move your fingers about 1/2 to 1 inch further up the zipper to allow for some extra room on either side of your credit card. You should end with about 5 inches on the end of the zipper.
Fold those 5 inches over the rest of the zipper, with the outside of the zipper facing in.
Turn the zipper so that the fold is on the left side and the opening is on the right.
Sew down the folded edge of the zipper (the left side) so that it stays shut and sew across the bottom of the folded section as well. The right side and top should still be open.
Grab the next section of zipper (the part just past the 5 inches you already sewed) and fold it over the right side of the zipper. Fold it in the opposite direction of the first fold so that this lays on top of the first layer and forms the second layer of zipper.
Sew the second fold to the right side of the original fold to close it up. Then sew it to the top of the first layer.
Continue sewing and folding your zipper so that each fold is the same length as the one beneath it and is sewn to its top. The zipper should be wrapped around itself to form a spiral, with each wrap one layer higher up than the one beneath it.
Finish your purse by taking the last 5-inch section and laying it flat, then sewing both sides to the layer beneath it. It's okay if it's a little bit longer or shorter than the layer beneath it. This should close up your purse.
Unzip the top of your purse, which is the last section of zipper.
Reach inside and turn the purse inside out so that the outside of the zipper faces out.
To ensure a perfect finish, measure your zipper before you start and find a width that divides evenly into that number. For example, a 60-inch zipper would work with a 5-inch fold, but a 64-inch zipper would not.