Requirements:
Bucket
Source of Water
Diamond Pickaxe

Steps:

1)Use your pickaxe to dig underground to find a source of lava

2)Use your bucket to collect the lava

3)Go back to the surface where you have your source of water

4)Take out your bucket and empty the lava in the source of water

5)Wait a few seconds

6)There will be an "Obsidian" block

7)Use your diamond pickaxe to break it

8)Pick up the Obsidian you now have a block of Obsidian

9)Keep repeating steps 1 through 8 until you have 10 to 14 blocks you can now make a nether portal from the Obsidian!

Note:Obsidian take a while to break so keep holding LT