To change a ballast in a shop light you need to follow a couple of steps to ensure its done safely. Shut the power off to the shop light from the breaker or fuse box. Remove the bulbs and bottom metal cover. Locate the wiring that is coming into the fixture and connecting to the ballast. Disconnect all the wiring and unmount the ballast. Mount and wire your new ballast in the same way you removed the old one. Replace the bottom cover and bulbs. Turn the power back on to your shop light.