How do I reach the Past?

Right-click an Emblem of the Past anywhere outside the Past. It drops you into the Past from y=200 with Slow Falling.

The Emblem is crafted from four Astral Ingots, four Nightmare Ingots and a Manifestation of Infinity. Both ingots are made from Universium, which needs a Dragon Egg, so the route runs through the Ender Dragon in Act IV. See Emblem of the Past.

To leave the Past, warp out with a Warp Stone or Warp Scroll, or die. The Emblem doesn’t work inside the Past. See Visit the Past.