Example Description
The missing Adidas YKK zip on this top is usually the result of someone deciding that the logo would look better dangling from there shoe laces, pendant or key ring.
Find a replacement "medallion". The type used for our repair in this example is by Opti. They are not quite as good as the YKK ones, but this all makes for a worst case example. You can't just fit the spare slider on in place of the broken one. The zips are not compatible and won't work correctly.
Use a good pair of cutters or pliers to dismantle the replacement. Be careful that your not going to cut through or bend the tag bits we need. Mind your eyes to.
You need to neatly remove the components from the garment slider. Here, we use a small drill to cut the crimped edges away. If these components are missing, you'll need to obtain some more from yet another YKK compatible chrome zip. This should be easier to find though as your not going to need one with any logo on it.
Left to right: The cover, ratchet spring, Ratchet pin and the spare tag in place on the zip slider. Use tweezers to drop the pin and spring in place. Mix some epoxy glue such as Araldite and apply a small dab to the areas of the cover that were crimped and fit the cover.
Replacing the whole slider is possible if your spare is the same type. If your spare is exactly the same you can prize out the metal zip stop at the bottom of the zip and fit the new slider. If the garment has a full length zip that separates, then the job is much easier as you can prise out the top zip stop. Then remove and refit the sliders.

If you need more information. A good place to ask is in the rec.crafts.textiles.misc newsgroup.