The sacrifice needed to go Beneath is great. First, hold a Scythe. Find a Pig, Goat, or Sheep. Then, obtain seven offerings. Valid offerings are skulls, anvils, placed gold, pig iron, or black steel ingots, and gold or black steel plated blocks.
Kill the animal within a 5 block radius of the offerings. A portal will be created, but be warned! The Beneath will temporarily invade your world, wreaking havoc and causing confusion.
For the more common types of sacrifices, that do not open portals to other realms, see the chapter on Ancient Altars.