Step1
Level up your character. Running the final bosses won't be easy, so you'll want a seriously powerful character on the Battle.net ladder. Get your level up to at least mid-70s and hopefully have a lot of strong gear as well. Having friends with strong characters is going to be a necessity, because the final bosses are quite brutal.
Step2
Prepare your character to gather the keys. The three keys (Hate, Terror, and Destruction) drop from three non-act bosses in Hell difficulty. Worse yet, they don't drop all the time. And worse yet, you'll eventually need three of each key. You will likely have to kill the same three bosses many times, but the charm is totally worth it. And besides, you were probably just running bosses anyway. So don't just make a mob-fighting character, because this character will need to be able to stand up against some serious bosses.
Step3
Run the Countess in Act 1 Hell. At the bottom of the Forgotten Tower you will find the Countess. This is likely to be your favorite of the three runs, not only because Act 1 should present little problem to your souped-up character, but because in addition to the occasional Key of Terror (your goal), she also drops runes with a chance at some decently high level ones.
Step4
Run the Summoner in Act 2 Hell. Sure, the Arcane Sanctuary can be a pain for certain classes (skelemancer), but at least the summoner is not a tough boss. Just keep knocking him down until you gather up three Keys of Hate.
Step5
Kill Nilithak in Act 5 Hell. This is the worst of the three, not only because his Corpse Explosion at Hell difficulty can easily kill you, but because his dungeon is absolutely enormous. If you've been avoiding the Hall of Pain area waypoint to run Pindleskin, it may be time to suck it up and grab the waypoint, because you do not want to run the full dungeon three dozen times. Eventually, he will drop the Key of Destruction. Get three.
Step6
Now the fun begins. Once you have all 9 keys, make a new game and invite any friends with very strong characters. Return to town and put one of each key into the horadric cube and transmute. This will open a portal to a demi-boss, either a souped-up Izual, souped-up Duriel, or Lilith (a souped-up and renamed Andariel). Defeating the boss will net you a body part.
Step7
Avoid quitting the game. Return to town, move away from your previous portal and repeat step 6. If you are in the same game, the portal will be guaranteed to get you a new body part instead of one you already have. Do this once more and you should have all three body parts.
Step8
Finally, put all three body parts into the cube in town, move away from all previous portals and press transmute. This will open the final portal to Tristram, where you will fight three uber-bosses at once, as well as a neverending supply of mobs. Defeating them will require every trick in the book and then some, so bring an army that can handle anything. Having a few paladins with super-high lightning resist will be especially handy for taking down Mephisto, which you'll want to do first since his aura will crush you otherwise.
Step9
Once you finally kill the final uber-boss, the hellfire torch charm will drop. Hopefully it's for a class you have a character of. If not, enjoy the other tasty mods until you can trade it for a torch of the proper class!
Enthusiast 
