I started with 2 tablespoons of butter and 2 tablespoons of all-purpose flour and cooked that down until it became a light-colored, thick paste.
Then, I slowly added in 1 1/2 cups of milk. Once incorporated, I added the rest of my cheese: 1 cup of sharp cheddar, 1/2 of colby jack, and 1/2 cup of mozzarella.
After everything was added, I stirred until the sauce was smooth without any clumps.