The basic trick is to use the locking pliers to clamp down just a smidgen on the top and bottom plates of the zipper slider body. Do both sides, work slowly so you don't make the sides too tight.
Here's the step-by-step instructions we created for iFixit's web site: https://www.ifixit.com/Guide/Repairing+Zipper+Slider/35496
Fabric items are 8% of landfill waste. How many jackets, tents, backpacks, luggage, and other items with zippers have been discarded when they could've been easily repaired with this simple technique?