How can I farm Guardians?

I have a nice ocean monument, all emptied of water. I am planning on converting part of it into a guardian farm.

How should I do this? There are very few tutorials (that I've seen) that explain a simple way of doing it.

My farm uses a tank to spawn, and cacti to kill them.

The tank only has source blocks at the top, and the bottom is wall signs. The guardians spawn inside the tank, then are pushed down by the water, out the bottom.

Under the tank is a water conveyor, pushing the guardians to one side. At the end of the water, there is a row of hoppers in the ground, then a row of cacti (zig zagging, because cacti).

The cacti sometimes destroy the item drops, but it works for me.

This wiki page has a fairly in depth guide to Guardian Farming.

The basic idea is that first you will need to locate an Underwater Monument (this is the only place where they will spawn naturally), and altering the area to trap the guardians for the slaughter. So far there are two methods, "manual" and "automatic". The "manual" method involves the player killing the Guardians directly, whereas the "automatic" method involves the use of Iron Golems.

The wiki has also linked a couple of videos, outlining a standard survival guide, and an "OP Farm" style guide.