To find the secret cow level you must first understand there is no secret cow level however there is a secret level called Whimsyshire with rainbows and unicorns.
To find it you collect items and make staff of herding and use it on Old Tristam Road.
The video linked is the best one to help you find all the items and does not spoil what the secret level is like. (not my own video)
- The Black Mushroom from Cathedral level 1 in Act 1 (cathedral gardens way point go backwards through)
- Leorics Shinbone from the fireplace in Leorics Manor in Act 1.
- Liquid Rainbow in a Mysterious Cave from a Mysterious chest in the Oasis Act 2. To enter the cave you must find a random spawn vendor in the Oasis.
- Wirts Bell from a vendor in the Bazaar Act 2. This item costs 100,000 gold.
- Gibbering Gemstone drops from Chiltara a rare spawn in the Frozen Cave Act 3.
- The recipe for the Staff of Herding drop of Izual in Act 4.
Build the Staff
- Using all the items found above, the recipe for the Staff of Herding, and 50,000 gold. (at blacksmith)
- Take the staff to the open red glowing crevasse on Old Tristram Road in Act 1, use the staff and accept the quest. (Follow right side up Tristram Road)