A lapped seam is used when you need one of the strongest seams you can make one a home sewing machine.
It is an easy seam to do,.
First, cut your pieces, the lapped over part is going to be your seam allowance, if you want a 1 inch wide lapped seam fold over fabric 1 inch.
You fold and over one layer to the wrong side and press, then other layer fold over to the right side, then interlock them, matching them with rights side showing same.
Then top/edge stitch one fo the edges, then flip over and top/edge stitch the other edge, and you are done.
This is a good seam when you need it to look good on both the outside and inside, then look the same either way.