The key to sewing a very flat patch pocket is where you place the interfacing and how the fabric is folded and sewn. Here's how to do it:

1.  Make 1/8 inch deep clips into the seam allowances to mark the pocket facing fold.

2. Cut the interfacing 1 1/2 inch x the width of the pocket, less the 5/8 inch seam allowance on both vertical sides.  Place the top edge of the interfacing on the wrong side of the pocket at the top fold line.  Serge the top raw edge of the pocket facing. Staystitch around the pocket  5/8 inch  from the raw edges. 

 

3.  Fold the pocket facing to the wrong side at the clips.  Slide a strip of  Fine Fusing Tape or Steam-A-Seam under the pocket facing edge and press lightly.  Remove the paper backing and press to fuse the pocket facing in place.  

  4.  Topstitch across the Pocket 1” below the folded top edge.  

5.  Press the seam allowances to the wrong side, favoring the staystitching slightly to the wrong side so it isn't seen from the right side. Lay strips of Steam-A-Seam on the folded seam allowances.  Press lightly.  Remove the paper backing.
 

For close up view of top pocket edge.

   

6. Place the pocket on a front, matching the dot on the garment.  Be sure the edge of the pocket is parallel to the grain line of the garment fabric. Check all four edges of the pocket to make sure it runs along the weave in the garment fabric.  Examine the top corners of the pocket.  If any seam allowances or stray threads are peeking out, tuck them down below the pocket's top edge.  Press the pocket to fuse it in place.

7.  Edgestitch the pocket.  Place the garment in the sewing machine with the pocket top toward you.  Leave a long thread tails, insert the needle where the horizontal topstitching from stitching the facing meets the pocket edge.   Measure along the top edge 1/2 inch from the top pocket corner and place a pin.  Begin stitching diagonally toward the pin.  At the top edge, pivot and stitch to the corner, counting your stitches.  Pivot at the corner and continue edgestitching around the pocket.  After you've stitched around the pocket and arrive at the remaining top corner, stitch across the top edge the same number of stitches that you counted on the first corner.  Pivot and stitch diagonally to the point where the horizontal topstitching meets the edge of the pocket, leaving long thread tails.  Your second corner should mirror the first corner.  Pull the thread tails to the wrong side and tie them off.