The only requirements is it should be mainly potatoes, and cook it on the hob! Apart from that, you can use whatever vegetables you have around (fresh, frozen, tinned, leftovers), plus whatever cooked meat you have (leftover from a roast, a spare chop, tinned corned beef, etc), and some stock (fresh, from a cube, or leftover gravy!) Cook until the potatoes are soft, and bash it about - you're looking for something more chunky than mash, but all mixed together and soft.
Yummy - mine's usually got carrots and the end of a ham joint....