1) Put the skirt on your slipcover. End the cording just short of the seam you want to put your zipper in. Then sew your welt cord down the seam with your cording foot. Cut the welt about 2" longer than the end of your skirt. Then open & trim out the welting.
2) Working from the right side of your slipcover, tuck the zipper under the cording & make sure you are not sewing on the teeth & that the zipper teeth are totally covered by the cording, top stitch in the ditch between the body of your slipcover & the cording. I use #3 zipper because it is easier to hide, more pliable, & as strong as the old nasty metal zipper that we used to use.
3) You will sew through the skirt & as you get to about 2" from the bottom of the skirt, you want to tuck the end of the cording back under the skirt plus fold the zipper back on itself about 3/8" (the length of the zipper pull tongue) because you want to stop the zipper just short of the bottom so that the tongue on the pull doesn't dangle below the skirt. Once you've sew to the bottom & reversed back a couple of times, go to the back of the skirt & tack the end of the cording & zipper again.
4) Now you are going to work on the other side. In my sample, the left side would be the arm section & now this side would be the back of your slipcover. The cording on the first section that we did would have been continuous across the top of the slipcover. This section is an additional piece of cording that will be tucked under the continuous cording at the top where they come together.
5) Fold the seam allowance under & stitch the other side of your zipper to this finished corded section. I normally start at the skirt section & sew up towards the top of the slipcover. As I get to the area where the 2 pieces of cording & the zipper meet, I cut the welting out of the casing so that it will fit smoothly under the other continuous welt that you have already completed. I then sew the cording & the back section of the slipcover across the zipper & under the section already completed. The working from the underneath, I sew over the end of the zipper & close up the hole where I'd tucked the zipper in.
6) After I get the top cleaned up, then I go back & sew from the skirt down & finish putting the zipper in. I usually tack across the zipper at the top & the stitching on both sections on the skirt just to make sure the slider doesn't come off the track.
No, this isn't the only way I put a zipper in a slipcover. I try to pick the installation technique to the fabric & the piece of furniture. And even though I'm the Zipper Lady, I try NOT to put zippers in slipcovers if possible.
Send me pictures of how you put zippers in slipcovers. I'll post them here.....