The Staff of Herding, and its respective upgrades the Nightmarish, Hellish, and Infernal Staff of Herding, are used to access the 'Secret Cow Level' in Diablo III dubbed Whimsyshire. Each individual form of the Staff is used to access the SCL in its respective difficulty.
- Normal Difficulty: Staff of Herding
- Nightmare Difficulty: Nightmarish Staff of Herding
- Hell Difficulty: Hellish Staff of Herding
- Inferno Difficulty: Infernal Staff of Herding
The Normal staff is obtained by crafting. You will need 50k gold, Black Mushroom, Leoric's Shinbone, Liquid Rainbow, Wirt's Bell, and a Gibbering Gemstone, as well as the plans to make it, which randomly drop off of Izual in the Normal Difficulty (The plans, not the other items). Note, that after defeating Izual in the harder difficulties, the plans may be bought from Gorell who is located within Bastion's Keep Stronghold.
The upgraded staves are crafted using the original staff (or whatever last upgrade applies) and additional gold, as well as the recepie, also obtained from Gorell.
- Nightmarish 200k gold
- Hellish 500k gold
- Infernal 1 MILLION GOLD (HOLY $@@T!!)
Once the staff is complete take The Old Ruins waypoint (Act I) and head west down Old Tristram Road. Hug the south wall. There should be a Skeletal Corpse and a "rift" in one of the crevices in the ground. If you have the Staff, the Ghost of the Cow King will appear with a quest above his head.
Sources:
http://diablo.wikia.com/wiki/Staff_of_Herding
http://www.diablowiki.net/Staff_of_Herding
http://www.diablowiki.net/Nightmarish_Staff_of_Herding
http://us.battle.net/d3/en/forum/topic/5152408516
Yes. Whimsyshire drops the same loot as act 4 of the same difficulty that you're on (except for inferno which drops loot of the same quality as acts 3 and 4, which have the same drop rates).
This means that Hell Whimsyshire will drop iLvl 61 and 62 items at the same rate as Act 3/4 will.
Best Answer
Since the plans are legendary items additional magic find can help them appear, from that point of view you can get 475% MF with MP10 and hit him in Normal or Nightmare.
Seeing as they can drop on any difficulty level, farming Izual/Diablo in normal or nightmare is a pretty straight forward way to get the plans.
With that said the plans are in a different bucket and appear far more often than other legendary items, so quantity of kills is ultimately what will get you the pop fastest.
All the other components can be farmed from the nearest waypoint in normal for extreme speed.