It's nice to bake one extra potato so they can be extra-stuffed, and as insurance against breaking one of the shells.

Ingredients Step
Preheat oven to 400 degrees Fahrenheit.
4 baking potatoes, scrubbed Bake until tender -- ideally in oven so the skins firm up, but microwave is fine too. If in microwave and they are normal sized, stab all over with a fork then microwave it for 5 minutes, flip, and 5 minutes more.
½ c. milk
1 t. Dijon
¾ c. cheddar, grated
½ t. salt
¼ t. pepper
Cut potatoes in half lengthwise. Scoop out the flesh. Mash it all together. Maybe whip until light & fluffy. Spoon into reserved skins.
¼ c. cheddar, grated Sprinkle.
Bake another 10 minutes at 400 degrees, to brown the tops.

source: my grandma