Milking It is the fourth quest given by Magnus, and it appears immediately after you finish the Selling on the High Seas quest. It's designed to get you up and running with a Cow Pasture, and is your first introduction to the joys of dairy farming in Castleville Legends. Trust me, you'll be doing a lot of dairy farming!
Note that the cow pasture resource is not available to purchase until you reach Level 6 in the game, so if you receive this quest before you have reached that level, you will need to be a little patient in completing it.
If you want to work ahead a little, keep producing your milk; you'll need it for the next quest, called Skimming the Cream.