If you are using leftover baked potatoes, skip baking instructions. To use leftover boiled potatoes, bake for 10 minutes in a 400º oven to brown skins.
Wash potatoes, prick all over with a fork, then bake until done. You can bake for about 45 minutes in a 400º oven if you prefer, or microwave on high for 6 minutes, then bake for 10 minutes in a 400º oven to brown skins. Let cool until they can be handled, or use hot pads and be careful.
Cut potatoes in half lengthwise and carefully scoop out the insides. Put the potato skins in a baking dish (any kind will work as long as it is big enough to fit all of them). Put the insides into a bowl (if you want to use a microwave) or a saucepan to make the filling on the stove.
Mash potatoes. (The back of a spoon is sufficient here; most lumps will disappear as the filling is stirred.) Add all other ingredients and stir. If you are using the stove, turn to medium low and keep stirring until cheese is melted and potatoes are smooth. If potatoes are still slightly crunchy, turn heat up to medium until they are soft. To use microwave, heat the filling on high for 1 minute at a time, stirring between each heating period, until filling is smooth. If mixture is too thick to stir easily, add extra milk or water.
Distribute the filling among the potato shells, then bake in a 400º oven until tops are browned (about 15 minutes).