Fold about 1 cm (0.4 Inch) wrong sides together of the bottom side of your fabric and Iron it. Do this on both pieces.
Then align your 2 pieces of fabric (folded sides together) and place your zipper on top as centered as possible.
Now, align your zipper from end to end to the folded edge on top of the folded side. Pin in place on both sides making sure the 2 pieces are still aligned.
If you turn it you will see the zipper is not visible.
My zipper was slightly larger than my pillow size so I centered it. Once you finish pining zipper in place you will get something like this:
The first thing you need to do is to put your zipper foot on.
I found out easier to work with the zipper unzipped. I mean all open.
Be sure you are back to a regular straight stitch and start at the end of the zipper farthest away from the zipper handle sewing just like you would any other straight line, but keep it as close to the zipper teeth as possible.
When you get to the zipper pull you won’t be able to keep going because it takes up some space. So stop just a little bit away from it, keeping your needle down in the fabric (if possible.) Lift your foot and carefully zip the zipper past the foot, lower your foot and continue to the end of the zipper. Backstitch to end your stitch.
For the other side, I do the same I did but I start from the other end as I mentioned. That way I keep the fabric always on the left side of the sewing machine. Am I too weird?
Your zipper is installed. Woohoo!
But before moving on we need a final touch to the zipper. Zip the pillow all the way shut and about 1 cm (0,4 Inch) from the fabric edge, stitch across the zipper along the edge for the beginning of the zipper and then open it slightly and align the zipper ends. Stitch again but this time across the zipper. Just a few stitches about an inch long onto the zipper.
After you zipped the pillow part way in the step above, you will want to cut off any overhang on the zipper track