1.dig deep(just deep enough so its under any other ground
2.expand it huge(dosnt have 2 be u choose the bigger the more mobs
3.dig little canals(2 bloxks wine 2 blocks down)
4.make sure u have platforms that r big for the mobs 2 spawn on
5.make the canals lead somewhere(dont need 2 tho Ex.have multiple elevators)
6.make an elevator(dont know how?learn at bottom of hint)
7.make air pockets so mobs dont drown
8.make it 2 the surface
9.put a base around it(make sure creepers dont see u LOL)
10.make glass
11.upgrade base 2 glass(watch em drown >:D)
12.make the base 3 blocks high(they wont fit in 1 block high)
12 layer 1 waterflow 2 item collection
12 layer 2 signs 2 hold water ontop(dont know how 2 use signs?look at bottom of cheat)
12 layer 3 a layer of water 2 drown them >:D

13.make the item passage 1 block wide and 1 block tall(no mobs fit!)
14.make sure ur on peaceful building it and turn it off when ur done
15.make sure there is NO lighting underground(mobs spawn in darness)
16.enjoy killing ur enemies >:D

help:make an elevator:mobs always swin 2 the top of water unless its superpull down.
To stop mobs from drowning and not floating up add air pockets with signs...

using signs:signs allow mobs 2 pass through but work as a full block for water
(for using this on the ground use preassure plates)

made by superandrewkilldude(youtube) credit to ethoslab(youtube) dosnt work for spiders...
spiders fit in 1 block tall spaces but 2 by 2 wide.ajust for spiders on ur own