There are two ways to do a Railroad or Centered Zipper: basted or open seamed. We’ll start with basted…
Sew a basting stitch at the seam line where the zipper is going to go.
Press seam allowance open
Open zipper and pin the correct side of the zipper to the seam allowance on one side
Sew, with a regular stitch, from the top edge 3/8 inch away from the zipper teeth to just past the bottom of the zipper, but not past the bottom of the zipper tape.
At the bottom of the zipper, with needle down, lift foot and turn zipper/project to sew across bottom of the zipper tape (put the foot back down to actually sew)
Then needle back down, foot up, turn to sew up the other side of the zipper, foot down and sew to the top
Take your seam ripper/button hole knife and pick out the basting stitches
And there you go.
Not hard at all.
And at this point I totally just realized I never did the open method of a railroad zipper today. H0-hum. How’s about we just leave this here as a marker and I’ll come back and update in a bit after I get that done, hmmm? In the meantime, the open method is pretty much EXACTLY the same, only instead of doing a basting stitch, you just skip to the pressing the seam allowance open, pin the zipper in on both sides (with the zipper open) and then go from there. It’s the way I usually do things. Super simple, but I’ll get back to you with pictures and stuff. One day. Promise.