A guardian farm is a contraption, that automatically spawns and kills guardians, and puts the drops into chests.
Guardian Spawning conditions Edit
Guardians spawn inside the 57x57 area of the monument from Y level 39 to 61. They require a 1x2x1 (x,y,z) area to spawn. When there is direct sunlight access to the water source the chance of them spawning is 10 times lower. That's why using a roof drastically improves the rates. It's also really good to use a non-solid block (glass, glow stone, slabs, or red stone blocks) since the new mob spawning AI starts off at a transparent block so it raises the rates quite a bit from my testing.
Processing Guardians Edit
Once the guardians spawn they need to be killed as quick as possible so I recommend having blocks next to each water source in order to get a downwards going stream. They need to drop 35 blocks in order to be killed. (34 will kill most of them but not all of them). A good video that explains these things and more is this one: https://www.youtube.com/watch?v=s0FiZ51A6Qg
(The information from that video should be extracted to this page)