Once you have the staff created, head to Act 1 and use the Old Ruins waypoint. From there head down the path to Old Tristram Road and then southwest. You will find the skeleton of a cow with a huge crevice near it. If you have the staff in your inventory, the Ghost of the Cow King will spawn with a quest mark above his head. Once you talk to him, he will go through his dialogue and remind you that there is no Cow Level. After which the crevice will turn into the entrance to Whimsyshire.
Note: not everyone in your party needs the staff to enter, just one person needs it to talk to Ghost of the Cow King.
All in all, it will cost you between 150k and 1.9 million gold (depending on how far you want to upgrade the staff), and some patience, but you’ll earn the feat of strength In the Land of Killer Unicorns and some nice loot for your troubles.
