1. Place right sides together and stitch around rectangle.
Line up the markings on the front of the garment and pocket piece. Stitch around the outside of the opening rectangle, paying careful at the corners so they are crisp and exact.
Measure as you sew to ensure your box is exactly where you want it; if you need to make any changes or redo part of it, do so before you make any cuts.
2. Cut up the center and into corners. Flip and press.
Cut all layers up the center, stopping ~1/4″ from each side. From there, cut into the corners as close as you can, without cutting the stitching. Pull fabric through to the back, and press well. Check to make sure it looks good from the front before moving on.
3. Fold up pocket piece to cover opening (create welt). Press.
From the wrong side of the garment (the wrong side of the pocket piece facing up), fold up just enough to cover the opening to create the welt. Press into place, and again, make sure it looks how you want from the front.
4. Stitch triangular seam allowance to pocket piece.
Once the welt is made, you have to secure it. Under the folds on the sides, find the triangle that was cut from the center. Stitch that to the front pocket piece fabric. Your pocket should be done and look good, and all that’s needed now is a back piece.
5. Add back piece of pocket and stitch around perimeter. Trim excess.
With the garment right side down (wrong side of pocket piece facing up), pin the back pocket piece to the perimeter of the front piece. The corners should all match, but if they don’t, make the back piece match the front piece.
Stitch around using your determined seam allowance, then trim any excess.