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
Step 9: Almost done!
And..... The mob farm is done!
BUT....... It can take some time until it starts spawning mobs...
But when its running it can produce up to about 1000 drops/hour.
To use the mob farm, all you have to do is to stand in the small room under the farm and whait, BUT..... do not stand right where the mobs lands, if you do that they can damage you
the drops on the second picture is after about 5 minutes