Minecraft wiki entry for Witch_farming, associated text: 1 Purpose
2 Mechanics
3 Designs
3.1 Simple
3.1.1 Preparation
3.1.2 Spawn Platforms and Flushing System
3.1.3 The Other Spawn Platforms
3.1.4 The Trench (Getting the Mobs into a 1x1 area)
3.1.5 The Entity Minecart Grinder Tutorial
3.1.6 The Tube style Fall Grinder Tutorial
3.1.7 Closing up the farm, spawn-proofing and the AFK Room
3.2 Selective flush design
3.3 Shifting floors design
3.4 Tripwire shifting designs