Do it like this lovely gentleman does do it:
But, heed this cautionary note from YouTube user Jammanen3; who so eloquently outlines a possible pitfall thus:
DON’T put that thing in the middle of the other thing. It broke my thing -.-. Use the thing on the sides of the sides of the other thing. That will work and hopefully not break the thing. :P Now I have to use some super glue to fix it -.-. Small pieces on the thing so it took me like 20 mins to fix it D:
Or in other words, don’t compress the top of the slider (the bit that actually meshes/unmeshes the teeth of the zip) with pliers, instead compress each side of the slider separately. Otherwise the piece of metal joining the pull-tab (take a wild guess..) to the slider can get mangled and come off, which you’ll then have to superglue back.
Fixed my shorts with a wonky zip double-fine :D
Although saying that, a few less pies and a bit more exercise and the zip wouldn’t have been knackered to begin with…