Step 1: Get some mesh bags
Step 2: Fold up the bag
In the folded photo, the edges on the right are the ones that will be sewn together. After you sew them, you'll turn the whole thing inside out to hide the seam, so the loose ends (on the left) are on the outside for now so that they will be on the inside when you finish.
(The photos are in reverse order so that this step's end result is the main image.)