He taught us a technique where you turn under the seam edges and create smooth rounded "corners" at the same time. He had us use this technique for the flap of a double welt pocket. I have since translated this same technique to patch pockets and have always been pleased with the result.
1. Cut out the patch pocket in your chosen fashion fabric (in the photos below, I used a thick canvas fabric). The chalk line in the photo below is where the fold will be. The area above the fold is the pocket facing. Also cut a pocket in a lighter weight fabric. The pocket piece in the lighter weight fabric should not extend beyond the fold line so as to keep bulk to a minimum.
2. Place the fashion fabric and lighter weight fabric right sides together. Per the photo below, push in the fashion fabric approximately 1/8 inch and pin all around.
3. Hand baste all around the pocket. You can then remove the pins.
4. Trim off the 1/8 inch lighter weight fabric that extends past the fashion fabric. The effect is to make the lightweight fabric, which will be the underside or wrong side of the pocket, smaller than the fashion fabric. This enables the lightweight fabric to pull in the seam so that the seam rolls under the pocket. The seam therefore will not be visible on the outside.
5. Machine sew with the fashion fabric side down against the feed dogs. The feed dogs will help ease in the fullness.
6. Trim all around the pocket. When you get to the corners, trim the seam allowance down to a 1/8 inch seam allowance. Having a small seam allowance is what ensures a smooth rounded shape in the finished pocket.
7. Turn the pocket right side out and press. See how the edges are rolling toward the underside of the pocket. You can use this technique for anything that requires that the seam roll to the underside, like a collar for example.
8. Finish the pocket facing whichever way you like. You can turn under the raw edge or serge it. I decided to turn mine under by 1/4 inch and then I topstitched it down. Also make sure that the sides of the facing turn in slightly so they are not visible on the outside.
9. Press and here is the finished pocket. The rounded "corners" are smooth.
In sewing, there are many different ways to accomplish the same thing. Some may prefer to make a patch pocket without going through the trouble of adding another layer underneath. Others may be adept at using those metal templates that help you press rounded corners. I have never had luck with those templates so I prefer this method.
There is no right or wrong way. There is only what works best for you. Happy sewing!