Don't let a bung zipper stop you from wearing your favourite pair of jeans (or indeed, any zippered item). Buying a replacement zip is cheap as chips and won't take you long to stitch on.
Before we dive in, it's worth noting that stubborn zips, zips missing a pull tab and those pesky zips that just aren't lining up, can often be fixed with a safety pin or a bit of creative maneuvering and lubrication. If the teeth on your zipper are damaged or missing, however, you'll definitely want to replace the whole thing. Sewing a zip via the method below should work on any unlined garment, whether you've got a metal or plastic zip.
Hand sewing needle
Colour matching thread
Note: if you are replacing a zip on jeans, you’ll need both a machine needle and thread that can sew through denim.
1. Unpick your original zip from the garment. On the front-facing side, there will be two sets of stitching to unpick. You might also need to unpick a few stitches from the waistband.
2. Measure the length of the original zip on your garment. It’s rare to find another zip that’s exactly the same length as your original, but don’t fret, it’s very easy to resize zips to the perfect length.
3. (Go straight step 6 if your zip is the right size!) For everyone else, mark your zip to the right length, matching the tape end on your original zip.
4. Using your machine, slowly sew a straight stitch across the teeth and go back and forth a few times to create a new stopper. If you have a metal teeth zip, use the manual wheel and maneuver the zip slowly to get in between the teeth and avoid breaking your needle.
5. Trim the zip end 1cm from your new stopper. If you’re working with a metal teeth zip, you may need to twist the zipper as you do this.
6. Pin the left side of your new zipper tape in place. You might like to baste the zip in place as well (hand sew a wide stitch to hold the zip). Machine stitch this down, then remove the basting stitch.
7. Pin the right side of your zipper tape in place.
8. Thread a hand sewing needle in a colour-matching thread. Taking care to only catch the fly facing (the under layer before the top layer of your pants), hand stitch the zipper tape. You can use a variety of different stitch techniques to get the best hold. I’ve done a little vertical stitch in the middle of the tape, and two little horizontal ones to hold the side of the tape down. Make sure to do a couple of small hand stitches to close up where you unpicked the waistband.