Firstly pin the seam you want the zip to go in as if you were going to sew it all the way up. Then using tacking stiches along the line of the seam, measure the length of the zip and place a pin perpendicular to the seam where you want the bottom of the seam to be (above the zip stopper). Sew the seam and press it open all the way up.
Now lay the zip over the selvage edges and pin and/or tack it in place (don't go through both layers). Now use a zipper foot on your machine or neatly hand sew the two sides of the zip to the selvage edges. Now you're done!
Now lay the zip over the selvage edges and pin and/or tack it in place (don't go through both layers). Now use a zipper foot on your machine or neatly hand sew the two sides of the zip to the selvage edges. Now you're done!