and melt. Once bubbling subsides, add flour and whisk vigorously. The roux will start chunky and gloopy. Keep heating over medium high heat and whisking very regularly, if not constantly. After a minute or three of whisking, check for how the roux flows . You should notice it becoming more liquid-like, as it'll flow more like a liquid and with less clumping like a solid. You now have a roux. You can take it a bit further, whisking constantly to take the roux to a deeper tan and impart more flavor, or you can keep it blonde where it has the most thickening power. SLOWLY add warmed (not hot) MILK (not cream). As you drizzle in the milk, whisk vigorously to incorporate the milk as you add it. Simmer for 5-10 minutes to thicken as needed, add a pinch or two of nutmeg, and you now have a bechamel. Slowly whisk in your grated cheese, a quarter cup or so at a time. You now have a mornay. Btw, in terms of ratios, they can vary quite a bit, but typically, you'll see 1 cup milk for every two tbls butter in your roux. As for the mornay, it's usually 1 part cheese to 3 parts bechemel, with the cheese being split half and half parmesan and gruyere. But hell, it's mac and cheese. It's forgiving. Get creative. Just stick close to those ratios and you can't lose. And for the love of god, don't use cheap, shitty, high-fat cheese