Add a Lining to a Lapped Zip
First of all prepare the top of the opening before inserting the zip.
This will be the side of the opening that will make the overlap.
Before sewing the lining seam
1. Machine single layer of fabric, The length of the stitching will be the same as the stitching for attaching the zip. One side is 15mm from the edge, the other 25mm from the edge pivoting and stitching to the edge of the fabric.
2. Trim approx 20mm and snip into the right angle