My dad is on long term warfarin therapy for a mechanical aortic valve. He had stopped taking it for a couple months for reasons unknown and desires to get back on it. What is the proper procedure in this situation? What kind of things can he expect from the doctor?

These are questions for his doctor. We are not doctors here, nor do we know his medical history. Any advice of this sort you get from strangers online can be detrimental to the health of your father.