slowly add 1.5 cups milk and vigorously whisk it into the butter and flour mix (slowly is key here too fast and you’ll end up with lumpy sauce)
Once all the milk is added you should have a smooth but thin sauce. Cook your sauce until it reaches a consistency that you want then add your cheese (I add 100-150g of parmesan to this for alfredo or 150g of cheddar for mac and cheese sauce). Remember that the sauce will thicken more once you take it off heat.
Edit: Also spices are on you. Spice it however you like starting with at least some salt since the milk will take some salt away from the cheese.