I really need someone who can draw to illustrate this! I will try, but no promises!
For a lapped seam the person must know how to use the stitching line as a guide for seaming.
Start with the stitching line clearly drawn on each piece, right sides up.
Add on 3/8″ seam allowance to your stiching line for a new stitching line.
Place one piece over the other, aligning on the new stitching lines and pin baste.
Stitch the stitching line
Grade seam allowance to 1/8″ on either side of the fabric.
With the graded seam allowance lying FLAT, take bottom side fabric and turn it over the seam allowance and pin to just past the other graded seam allowance.
With the graded seam allowance lying FLAT, take top side fabric and turn it under the seam allowance and pin to just past the other graded seam allowance. Now both pieces have reversed directions
Top stitch either side of the lap down.