Instructions
Search your "Minecraft" landscape for rare, naturally-occurring obsidium deposits near lava pools in caverns or dungeons, or divert a water stream into any lava source to create new obsidium. Each water block that meets a lava "source block," or non-flowing lava block, produces a single obsidium block.
Press "I" to open your inventory and equip your diamond mining pick. Face the obsidium deposit and hold down your left mouse button until the deposit collapses, leaving a collectible resource block on the ground. Walk over the block to pick it up and add it to your inventory. Repeat this process until you have collected 10 obsidium blocks.
Open your inventory and equip your stack of obsidium blocks. Select a location for your portal and imagine a grid in the space with four blocks per side. Right-click to place two obsidium blocks in the second and third spaces of the bottom row. Place two blocks in squares one and four of the second and third rows, and the final two blocks in spaces two and three of the top row.
Press "I" to open your inventory window and drag one iron ingot and one piece of flint to the first and fourth squares of the crafting grid in the upper-right corner, respectively. Drag the resulting "flint and steel" from the output box to your inventory.
Equip the flint and steel with your scroll wheel and right-click the inner surface of any obsidium block in your portal. When the inside of your portal appears purple, stand in the center to teleport to the Nether.