Apply a small amount of lubricant to the keyway
Use needle-nose pliers if any part of the key is sticking out
Use tweezers to grip a visible fragment
Insert a broken key extractor tool and hook the fragment out
Slide a thin jigsaw blade or hacksaw blade alongside the key to catch the teeth
Use a strong magnet if the key is magnetic
Tap the lock gently to help the fragment shift
Turn the lock slightly if possible to align the broken piece
Remove the lock cylinder if the fragment will not come out
Call a locksmith if the key is stuck deep inside