Tutorials/Witch farming

Witch farms are built in Witch Huts and are an easy and plentiful source of redstone dust, gunpowder, glowstone dust and other useful drops. The simplest witch farm involves making two 9x9 platforms with 3 air spaces between them; the uppermost at one block below the roof of the witch hut, and fencing an iron golem into each platform. The area for spawning witches includes the balcony to the witch hut. The platform can be made from hoppers to facilitate item retrieval.

Locating a witch hut
The easiest way to locate a witch hut, if you have access to the world data files, is to use Amidst 3.6 or later to view the map. See the forums for more information.

Designs
The following video is an advanced witch farm for well-established worlds with plenty of resources