Step 1: Extra Instructions
This spawned will only work in survival, it can be created from any material, it will make mobs fall into the water and make them fall and die, then you can collect the stuff!
Step 2: Make a Base Like This
At the end of making this, you can put water in the space behind there to bring the items to you.
Step 3: Stack
Keep on stacking for 18 blocks up
Step 4: Platform
Start to build the platform
Step 5: Finish
The finished product of the platform, go back to signs for measurements.
Step 6: Build
Start making walls like this, don't fill the walls and roof in.
Step 7: Build Up
Build up the wall only 2 blocks high.
Step 8: Build a Cross
Build like this, so it makes a cross with the middle as the center of the cross.
Step 9: Signs
Place signs like this, this is very vital to the next step.
Step 10: Water
Place water at the edges of the cross, this is where the mobs will fall and then fall and die.
Step 11: Fill In
Fill in the roof and walls.