The surfaces that are between teeth are not accessible to brush; therefore, the best way to clean them is by flossing. The frequency of flossing is like brushing and ideally after each meal, though one time a day (before going to bed) is the minimum necessary.
To start, cut a piece of dental floss (approximately 18 inches). Wrap both sides of the floss around your middle fingers. Using your index and thumb, glide the floss in between all your teeth one by one. When flossing, make sure you are not cutting your gums. The goal is to clean the teeth surfaces, not the gums. In the space in between teeth, press the floss against each side of the tooth (hug the tooth) and gently move it back and forth and up and down. Then move to the opposite surface of the adjacent tooth. As you move on to each tooth, unwind the floss from your fingers, and rewind it so there is a clean section of floss to use.
When you have finished flossing, follow with brushing and rinse with either water or mouthwash. Try to floss before every brushing. If you do not have time for flossing in the morning, always floss in the evening before your final brush of the day.