Clean the potatoes.
Skin the potatoes.
Put the skins to one side for a different recipe.
Quarter the potatoes and put a pan of lightly salted water on the cooker but don't turn the cooker on.
Add the potatoes and turn on the stove. (as a rule, if it grew above the ground put in hot water, below put in cold)
When the potatoes are nice and soft, pour the water (carefully) into a cup or jug. Store the potato water in a jar or bag in the freezer. Starchy water is AWESOME for making pastry. (Or I guess if you want gravy, you can make that more immediately.)
Heat the pan again with a little oil. Add the potatoes and mash them up. Add a little milk and mash some more. It will go really soft and you can shape it into balls.
But by "a little milk" I mean, be ginger with it... as my Mum says "You can always add more, it's harder to take it out"
If you're a bit funny like me, fry up some veggies like mushrooms or onions and add them... Or you can do the same thing with sweet potatoes.
My favourite variation is with the Sunday Roast's left over roast potatoes.