Lapped seam

Transfer the seam lines on to the wrong side of the leather layers. Cut off the seam allowance of the top layer and lay the wrong side of the top layer overlapping the seam allowance of the right side of the bottom layer. The raw edge should exactly align with the marked seam lined of the bottom layer. Secure the leather layers temporarily, using textile adhesive or leather cement. From the right side of the project, topstitch  close to the cut edge and again a presser foot width away.