First, find yourself a very large pot with a thick base that will distribute the heat evenly, preferably a stock pot. If the bottom is thin, consider some sort of diffuser. If you don't have one, buy one. They are cheap and you'll be glad you did. They even have them in the grocery store sometimes, for less than $10. Pour in your milk and turn on the heat to medium/high. Bring the milk to a boil over high heat stirring constantly, controlling the foam against boiling over.