Prep The Pants Before Attaching A Zipper
We begin by having the pants front sewn together at the crotch seam. I marked a 3/8" line on the left side where the zipper will be attached. Pin the fabric under at the line and baste it. You will later be removing these stitches.
Attach the Zipper
Pin the zipper to the left fly, with the fold of the fabric just touching the left side of the center part of the zipper. You can look at a pair of jeans as a reference if you need to.
Cut out the zipper placket, (a downloadable pattern for this is located here). It is a rectangular piece of fabric folded in half, to baste behind the left side of the zipper.
You can trim this placket to fit your pattern, if needed. Baste the placket behind the zipper on the left side of the pants.
After you baste through the left side of the pants, the zipper and the placket, you can remove the pins. Using a zipper foot on your sewing machine, sew through all the layers about 1/8" from the edge of the left pants, near the zipper.
Putting the regular sewing foot back on the machine, turn the pants over and add an additional stitch to just the zipper and placket, not stitching the pants front to it. This will help reinforce the zipper.
Reinforce Your StitchingNext, pin the right side zipper facing to the right front of the pants along the straight center edge. Stitch along 1/4" from the edge. Flip the facing to the inside and iron the fabric flat. Now you'll pin the facing to the front along the curved edge. Baste the zipper opening closed along the front center edge. Sew through all layers of fabric, basting the right and left sides together along the center.
Pin the right placket's curved edge to the pants front. and remove the basting along the center line. Open the zipper. Now you'll topstitch on the right side, following the dots marked on the outside (if you're following the McCalls pattern), mimicking the curve of the zipper facing. Be sure you do not stitch through to the left zipper placket - this will prevent you from being able to open the pants!
Reinforce your stitching at the bottom dot and the bottom center where the right and left sides come together with a bar tack stitch. Read about Bar Tack Stitching here