1. Find lava.
2. Take water on the lava.
3. You will se black stones (obsidian.)
4. Take 14 of them with a diamond pickaxe.
5. Then make this sqare of the black stones (obsidian.) -------> oooo
                                                                 o o
                                                                 o o
                                                                 o o
                                                                 oooo
6. Right-klick with a lighter on two black stones (obsidian.)
7. Then it will be a portal.
8. If you walk in the portal you will come to underworld.