Things You'll Need
Instructions
Lay two obsidian blocks side-by-side so that they touch. Place a dirt block on either side of the obsidian blocks to make a row of four blocks in total. The outer dirt blocks don't contribute to the portal, but they provide support for the sides.
Stack three blocks of obsidian on top of each of the outer dirt blocks on the bottom row. At this point, you have two obsidian blocks making up the bottom of the portal and three obsidian blocks apiece for each side for a total of eight obsidian blocks.
Target the inward-facing side of one of the top obsidian blocks of either column. Place a dirt block adjacent to it and place another one off of the opposite column's inward-facing side. This effectively connects the columns with dirt and provides a support for the top of the portal.
Place the final two obsidian blocks on top of the two connecting dirt blocks, and then remove the supporting blocks.
Strike the obsidian anywhere with a flint and steel tool to activate it, creating a purple plasma in the center of the portal. Walk into it to teleport into the Nether.