Place potatoes in a pot. Cover with cold water (starting with cold water will make sure your potatoes cook evenly all the way through and you don't get soggy outsideand salt water well with a teaspoon or two of salt. Bring to a boil over medium-high heat.
Boil until potatoes are tender enough that you can easily smash a piece of one against the side of your pot with a fork, about 6 to 10 minutes. Remove from heat and drain potatoes.
Using a fork or potato masher, smash potatoes in the pot. Add butter or margarine and a splash of milk or cream. Mix and add salt and pepper. Taste and add more butter/milk/salt if needed.