Place a pocket welt piece on the front (RST) with one long edge touching the slash. Mark the ends of the slash you measured in the last photo. Now place the second welt piece touching the other and mark the same. Mark 3/8 inch past each slash mark and draw a line 1/4 inch from the slash on both welts.
Now stitch along the marked line from the wider marks on each piece. I always count my stitches doing this as you don’t want a crooked pocket at the ends.