Add medium-high heat then add the flour. Stir for a few minutes, while the flour cooks a little and smooths out, then add the garlic and cook a further 2 minutes.
⅓ Cup All Purpose Flour, ¼ teaspoon Garlic
Slowly start to add the heated milk, stirring constantly. A little at a time, stir stir stir, a little more, stir. This takes time, if you rush it you will end up with lumps in your gravy.
3 Cups Whole Milk
Once you have added all the milk you should have a nice smooth gravy. Season with salt and pepper.
1 teaspoon Salt, 1 teaspoon Black Pepper
Notes
Depending on if you used bacon or butter to make your roux, will determine how much salt you need to add. I would definitely taste the white gravy before adding the salt so you know how much to add.