Minecraft wiki entry for Guardian_farming, associated text: Drain the temple
Create a large, flat room
Put a single layer of water over every block in the room, flowing to a single point to be optimal
Put a hopper at the point(s) that the water flows to, connected to a storage system
Activate a conduit directly underneath the center of the room. if there are no hoppers there and the floor is made of prismarine, the floor could be used as the top of the activation frame