Centered zipper application is the easiest of all in my opinion closely followed by invisible zipper. But... sewing a ripple free zipper on a side seam has always been a challenge for me. A solution is to move the zipper to the back seam, which you can certainly do but for me... I find that position uncomfortable when I sit with my back pressed against a chair back. So... with trial and error I came up with this fool proof method of zipper application on a side seam and so far it hasnt failed me. It's almost as invisible as an invisible zipper application. Ok, enough talk.. lets talk method :)
In this method, its easier to put the zipper in the side seam first and then sew the inseam.
First I baste the side seam closed. After the zipper mark I continue sewing the side seam with smaller stitches, backstitching after the zipper end mark.
Then I turn the pant inside out and place the side seam on a pressing ham, since the side seam is curved this works better. Press the seam open.
I take a zipper and stick double sided steam a seam on both sides of the zipper tape.
Then I carefully place zipper (face down) on the seam so that the coil was on top of the seam. We are still working on the wrong side of the pant.
Then I press the zipper so that it sticks to the seam allowance.
Turn the pant right sides out and topstitch on both sides of the basted seam allowance, making sure that you are catching the zipper tape. Use a zipper foot so that the sitches are even and stop after a few stitches to straighten the fabric so that there are no ripples. Another very important thing to keep in mind is to stitch in the same direction (top to bottom) to avoid any fabric stretching. so basically you are stitching a big L on both sides.
Give it a final press to set the seam.
Stabilizing the seam allowances with interfacing is also a very good idea.
Update: Ardis (SewVeryTall) says that the vertical seams should always be sewn from hem to the waist, so keep that in mind when sewing the pant seams. I will try this myself next time. Thanks Ardis!