Place the potatoes in a pot large enough to fit them all and cover them with just enough water. Bring to a boil.
Cook the potatoes until they reach an internal temperature of 2 degrees below the boiling point of water in your area (that changes based on elevation). If you don't have a thermometer then the potatoes need to be tender enough that a fork goes through them, but a thermometer is going to be most accurate. If cook the potatoes too long past this point they will become waterlogged and fall apart eventually.
Remove from water.
Place the potatoes into a large bowl. Just smash them with a potato masher to break them up. Then begin whipping them with a hand mixer. Whip for 10 seconds. Then add a little bit of milk at a time until you get the right consistency and continue whipping. Try not to over whip them - no more than 1 minute total with 30 seconds being ideal.