Step 1: The materials:
A flat area to build it on
4-5 stacks of cobblestone(or any other material)
1 and a half stacks of trapdoors
1 diamond pick, or 2 iron, or 5 stone
1 iron shovel, or about 2-3 stone
1 stack of ladders
some kind of house unless you swich to peaceful while building.
Step 2: Start building!
Build the walls 3 blocks high.
Step 3: The Roof
Then, you should probably put some torches inside
Step 4: The channels
Then, dig another one 1 block in from the first channel, and continue to do that
until you have reached the middle of the room
Step 6: Continue Diggin down
Then, dig straight down for 25 blocks and make a small room.
Step 7: Diggin UP
When you are up at the surface, youll probably need to mark where the hole is, i did this with a small "house"
Step 8: Trapdoors
When that is done, open all the trapdoors