Listen to the professionals using the harsh death metal vocals repeatedly. You must be able to hear the technique used by others before you can master the harsh death metal vocals yourself.
Drink the right fluids before you begin practicing your harsh death metal vocals. Warm water or milk are good for softening your vocal chords and preparing them for practice.
Start small. Try to emulate bands that are death metal, but not incredibly harsh at first. This helps you ease into the harsh death metal vocals without straining or injuring your vocal chords.
Use your diaphragm and breath as you sing. When you try to use your vocal chords only, and sing from your throat without proper diaphragm breathing, you can seriously harm your vocal chords.
Tighten your throat and vocal chords and push evenly with your diaphragm. Press the air from your lungs evenly through your tightened tonsils and you should produce a low grunt in your throat.
Add lyrics to this low grunt and you can practice intensifying the grunt. Again, ease into the harshest sounds to avoid straining your vocal chords and causing damage.