Edit (June 19,2013): Please keep in mind this is a very basic "how-to" for how Ubers are done. Most of the information is SLIGHTLY outdated (Although, should still apply 99% of the time), because the thread was created over 7 months ago. Thanks :)
For this guide I will assume that the reader knows absolutely nothing about the Uber Boss process, because I'm sure there are some people who do not.
~~~~~ Table Of Contents ~~~~~
2--> Uber Requirements / Suggestions
3--> The Fights
4--> Reader Q & A
~~ Getting the Keys ~~
In order to begin fighting Uber Bosses for the ultimate goal of getting a HellFire Ring, one must first make "Machines". These machines are made by gathering keys from Acts 1, 2, & 3, and making it using a plan found in Act 4.
To get the keys, you must kill the designated "Key Warden" in the first 3 acts. Each warden is unique in fighting style, appearance, and difficulty. Each warden also drops a key unique to that act (meaning key drops are not "random" in the sense that in order to get the Key of Destruction, you MUST kill the Act 1 Keywarden).
Following the rules of MP level, the wardens will have a better chance of dropping the keys at higher MP levels (10% chance at MP1, 20% chance at MP2, 30% chance at MP3, etc... etc...). It is suggested for most people to do "key runs" at MP 2 or 3 (if you can), because MP0 and MP1 require a fair bit of luck or a fair bit of trying to get a key.
Note: Always get 5 stacks before attempting to kill the wardens.
Act 1 Key Warden:
Location - Fields of Misery
Difficulty - 3/10 (relative to the average elite pack [assuming the average elite pack is a 5/10])
Special Attacks - Jailer/Molten - he also throws spears and launches a circular radius of fire balls at all players.
Key Name - Key of Destruction
Act 2 Key Warden:
Location - Oasis
Difficulty - Differs --> Range Chars - 10/10 ..... Melee Chars - 4/10
Special Attacks --> Vortex/Mortar/Electrified/Missile Dampening - He also spins and reflects projectiles back at players
Key Name - Key of Hate
Act 3 Key Warden:
Location - Stonefort
Difficulty - 5/10
Special Attacks --> Teleport/Frozen - He doesn't cast the standard frozen, he instead drops frozen bombs from the sky that don't actually freeze you, but slow you and deal substantial damage
Key Name - Key of Terror
Act 4 Plan Keeper:
Location - Silver Spire Level 1
Difficulty - 9/10
Special Attacks - Jailer/Knockback/Nightmare - He also casts a curse attack that reduces all healing effects (regen/LoH) by 75%. He can also cast a large circle of fire that hurts anyone who is inside it.
NOTES: Only need to get the Machine Plan ONCE, and beware of this warden. Suggested to run on MP 1 or 2
Warning: Act 4 is NOT friendly to anyone. Beware of hard mobs and difficult elites on your way to getting 5 stacks for the Keywardens.
Once you have all 3 keys + the plan, teach the plan to your blacksmith and the Infernal Machine should be under the weapons tab.