That's actually pretty simple if you have a sewing machine and a serger. You split the seams on each side by pulling the stitches – do NOT cut the seams of your garment and do NOT pull the sergered edges. Then, you make your pocket pouch: Begin by serging all the edges. Stich the pocket together, leaving an opening on one side where you will affix the pocket to those open seams on the garment. With the pocket inside out so that the serged edges are visible, stitch the opening of the pocket to the opening in the seam of the garment.

Be sure to reinforce the stitching at the top of the pocket where it attaches to the garment. This is because the pull and weight of your hand going in and out of the pocket will stress the garment at that point and could lead to small rips.