Just do a simple roux = 4 oz. (1 stick) melted butter, (< 1/4 c.) flour, (1/4 c. to 1/2 c.)milk or cream, salt and pepper, then add 1/2 of the cheese of your choice on really low heat, until melted, add to pasta then put in pan. Add 1/2 of remaining cheese to top for baking, then add the other half before serving.
Variations that are tasty: onions, mushrooms (or cream of mushroom soup), chopped fresh tomatoes, etc... As you like it!
Trust your instincts. You are a fantastic cook!!!