Just like in a pan! Use a pyrex jug. Melt your butter for a minute or so in the microwave. Stir in your flour. Add your milk bit by bit, stirring it. Whack it on medium power in the microwave for a few minutes. Stir it. Whack it in for more - watch that it doesn't boil over (so don't fill your jug too full). Stir it. Add cheese, s&p, grainy mustard, herbs if wished. Use.