Start slow on a metronome, once you can play it PERFECTLY even turn the speed up another 5-10 BPM, and keep doing that until you reach your desired speed.

Remember, Perfect practice makes perfect.
Practice, I would say, obviously. Also, try learning some Black Metal, as it is a very common technique of the genre. Pretty much any Emperor or Mayhem song, for example, will have tremelo picking.
when i want to practice tremolo, i just grab my pick and i start lol
first, it doesn't matter if u do it slowly. You only must do it fine.
You can increase the speed only when you think you can.