Instructions
Scroll through your inventory until your character holds obsidian.
Right-click where you want to place the obsidian. To properly create the portal, you must place the obsidian so it forms a 2 x 3 block frame.
Scroll through your inventory until your character holds flint and steel. If you do not have one of these, you can right-click any crafting table and make one. Place a piece of flint in the center square of the first column and an iron ingot in the bottom square of the center column.
Right-click the inner lining of the frame. The flint and steel lights the frame on fire, changing the middle of the frame to display a translucent purple wall. Stepping inside the purple transports you to the Nether.