I know of two methods to sew the patches to the foreparts. One involves completely prefabricating the patch, laying it on the forepart, then sewing it to the forepart from the wrong side with a diagonal stitch by hand. Matching the patterns here is easy. The seam allowance in this way of make is hidden between the patch and its lining (if it has one). The other method involves sewing the patch on the forepart with a sewing machine. If you think about it, this is impossible since the seam is in the inside of the pocket. Yet, tailors routinely do this in this part of the world, there is a technique to it. This way of affixing the patch does not allow for 100.0 % pattern matching. There is a tolerance of maybe +/- 2 mm. In this way of make the seam allowance is in the pocket space -- you can feel it with your fingers.