Add milk gradually and keep whisking continuously to make a lump free mixture.
Add chicken broth and cook until the sauce thickens.
Add parmesan cheese, garlic powder, salt and black pepper and cook until cheese melts.
Use the sauce as required.
Video
Notes
Make sure there are no lumps after you add milk in the flour. To do this use a wire whisk vigorously to mix while pouring the milk.Chicken broth adds a nice flavor to the sauce, so make sure to use a nice one.Use a good pan, it will ensure even cooking of the sauce.