Remove any pullers/toggles/fasteners from the zipper puller, and set them aside. These can be reused later.

Find a new slider, matching with the zip tape size. Craft shops, sewing shops, or even second-hand shops are great places to find these.

Tip: to determine the zipper puller size, check for the number on the backside. For more information, check out the iFixit guide Plastic Tooth Zipper Size

Take your new zipper puller, and insert it back onto the zip from the topside...

... and pull it down to the middle of the zip.