Step 1: Find a Place to Build
You should find a place to build far from your creations because it blows up a lot of stuff.
Step 2: Building the Frame
Next you can build it 4 or more high but leave to blocks in the middle empty like shown.
Step 3: Filling the Hole
Now you fill the hole with one block of lava in the middle and then place a tnt block I front of it.
Step 4: Ignite
Now just ignite any block you want and watch the explosion!
Step 5: Aftermath
That's a huge crater! If you don't believe me try this.
This also works on Xbox 360 and PC minecraft!