I don’t know if this is the “right” way or not, but every time I try what it says in the pattern I end up unpicking it and doing this instead. This way always ends up flat and no puckering.
First zigzag or overlock stick the edges of where the zipper will go. This way all the threads are out of the way.
Second, pin together the fabric where the zipper will go matching up seams and notches. Mark where the bottom of the zipper will be. I usually lay the zipper along the seams and use two pins to mark the spot (right above the metal end). Most times I will back stitch and switch to a regular stitch to finish sewing down the rest of the seam to sew pieces together. Even if the pattern has you do this separately, it lays flatter if you sew it all as one seam.
Third baste together the fabric pieces. Baste stitch is your longest length stitch. You just want to hold it for a while, but you are going to take this out so you don’t want a regular stitch here. I make the seam 5/8th inch in from the side so you have enough room on each side to sew the zipper in, but make sure your pattern allows for that before you sew.
Then press open the seam. and lay your zipper so the middle of the zipper matches your seam. Pin zipper on both sides.
Then using that same basting stitch (largest regular stitch) and using your zipper foot, sew along each side of the zipper attaching it to the fabric edge. Do this on each side with zipper closed.
Then on right side of fabric, using zipper foot and a regular length stitch start on right upper side of zipper and sew down right side keeping stitch about 1/2 inch from middle seam and zipper middle. You will not be able to see the zipper since you are sewing on the outside or right side of the fabric, but the seam in the middle should be the same as the middle of your zipper. Don’t sew too close to the seam or you will be too close for your zipper to go up and down. Also keep fabric straight and don’t pull as you go. This will keep it flat and even. At the bottom (right before metal end of zipper) pivot and sew across zipper to left side. You can pivot by making sure needle is down, lift up foot and turn fabric 90 degrees. Sew over end of zipper and do again. Make sure you don’t hit the metal end of zipper with your needle or it will break. Yes, I have done this too. Now sew up left side of zipper keeping same distance from seam to where you are sewing as right side.
Now with a seam ripper CAREFULLY remove basting stick down the middle. If you slip you will end up with a hole in your fabric and threads that catch into the zipper. Yes, I have done this before and it sucks, so don’t slip. Do it carefully.
Then remove all the threads that are along the edges.
You are done!! Well make sure zipper does go up and down all the way. I usually iron it after this step.