Use any seed you like or leave it blank. Where you spawn, walk to the closest tree.
Collect the saplings because you will be needing it. The very bottom tree block, dig down some blocks until
you find stone. This is where your venture starts! You will also be needing torches and trees.
From then on you can not go up, but if you want, you can find coal first and then go underground,
or just give yourself 32 coal. Make sure to leave a picture for what you have done in your underground cave!
Note: You can do this in PE, PC or Xbox. Once I did it in PE, it worked really well! Unfortuntely I lost it.
Remember food! You may play on peaceful. You may cover the hole you went down in. You may do anything you like to your mini underground dreamhouse!