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 7x9 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 is the roof overhang area and balcony. The platform can be made from hoppers to facilitate item retrieval.

In theory, a Witch farm could be built in 1.8 either using an Iron Golem to separate Creepers from mob farm spawns, followed by a Sun room to kill both Skeletons and Zombies; or a Lava blade trap to kill all but Witches, as they have the power to drink Fire Resistance potions.

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

Designs
Witches have more health than most mobs, and so require a drop of 32 to be absolutely sure of killing them. Their potion resistance and use makes them difficult to farm as a player kill. Drowning and burning are negated by potions, and crushing negated quickly by self-heal.

As most of their farms are on the world surface, environment conditioning can be avoided to a large degree by using a high-y AFK position. If you AFK about 124 blocks almost above the farm, adjacent to, but not in, it's chunk, it reduces the surface area available for mobs to spawn in. However, ensure the farm chunk has no high-altitude structures in it, and that your lighting at that altitude also doesn't enter the farm chunk, or your drop rates will drastically reduce.

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