You need to get 9 keys, 3 each of Terror, Hate, and Destruction from The Countess, The Summoner, and Nihlathak in Hell. These will allow you to open 3 red portals, where you will fight Uber bosses to get three more special items. Putting these 3 special items in the Cube will open the portal to Uber Tristram.
Here are the details from the wiki
The first step is to gather the three keys required to open the first
of the red portals. The Keys of Hate, Terror, and Destruction are
required, and can only be found from certain boss monsters, and only
on Hell difficulty. Get the Key of Terror from The Countess, the key
of Hate from The Summoner, and the Key of Destruction from Nihlathak.
It's a good idea to have 3 of each key before beginning the second
stage of the Pandemonium Event, so you can do all three of the special
areas in the same game. (The same area will not repeat in the same
game, so you'll have all 3 ingredients for Uber Tristram.) The keys
look identical, and can only be discerned between by hovering on them
to see their name.
Key of Terror These are dropped by The Countess, who is located on the
fifth level of the Forgotten Tower in the Black Marsh, Act One. She is
commonly run for her rune drops, and will drop a Key of Terror about
8% of the time she dies on Hell difficulty.
Key of Hate The Summoner drops this one. He's found at the end of one
of the four points of the Arcane Sanctuary, in Act Two. He will only
drop a Key of Hate on Hell difficulty, and even then only every few
Key of Destruction Nihlathak sometimes drops this key, but only on
Hell difficulty. He's found at one end of the four-pointed Halls of
Vaught, the lowest of the three Halls levels reached through Anya's
portal in Harrogath, in Act Five.
Opening the Red Portals
Once you have the keys, stand in Harrogath, on Hell difficulty, place
one of each in your Horadric Cube and convert them. A red portal will
open to one of three locations; the Matron's Den, the Forgotten Sands,
or the Furnace of Pain. Which portal opens first is random, but if you
do all three in the same game you will not get the same one twice. For
this reason it's recommended that you have 9 keys to begin with, 3 of
each type, so you can do them all in the same game. It's also
recommended that you stand in different places so you'll spread out
the three portals, so they don't overlap and make selecting the
correct one 'uber difficult'. You'll be adding a 4th portal once
you've completed these three, to Uber Tristram, so save room.
To create a red portal to Uber Tristram, place Diablo's Horn, Baal's
Eye, and Mephisto's Brain in the Horadric Cube, and transmute them
while standing in Harrogath in a Hell difficulty game. A red portal
will open, the fourth in that area if you've done them all in the same
game. It takes you to Uber Tristram, where the real fun begins.