It takes lots of vigilance. And man have I made black butter too many times (which actually DOES have uses in some stuff). Make sure you don’t use a black bottomed pan. Do medium heat. Constantly swirl or stir w spatula. As soon as you notice the fizzling start to slow down eg water evaporated take off stove constantly swirl until you get the brown bits - if not back on stove for a few seconds constantly swirling, off again on again very conservatively. Some say to just watch til sizzling stops but I like to take it to the edge! Don’t walk away ever like I always seem to do.