1. The Ingredients
For building a Nether Portal, you will need:
-14 Obsidian Blocks 
-1 Lighter 
2. The Positioning
You will have to make a rectangle 5x4 with the blocks vertically and to light the blocks below with the lighter. It will make a construction like this:
2. A Video
Here is a little video that will explain you how to build a Nether Portal:
