A food mill or ricer ensures a smooth puree. You can use an electric mixer as well. A potato masher or a fork takes more elbow grease but can do the job. The food processor is too hard on potatoes, leaving them with an unappetizing gluey texture. Baking potatoes make the best mashed potatoes. Mashed potatoes are best when served at once, for they lose their fluffiness when held or reheated.
Add and incorporate your butter/fat first, then do the milk or cream. Use butter, cream, salt, pepper and/or whatever you want, to taste!