How do I get into the Nether?

Use a Seal Of The Underworld: hold right-click with it for 5 seconds, and Nether portals start working for you. Until then a portal only shows a message.

The seal is crafted from the four seal shards, the Nether Crystal (a 50% Warden drop) and four Bottled Nether Essence. Act II’s quests lead to each piece; see Act II – First Steps and The Seal of the Underworld. The seal isn’t used up, so a team can share one.