The surfaces that are between teeth are not accessible to brush; Therefore, the best way to clean them is with 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 2 feet). Wrap both sides of the floss around your middle fingers. Using your index and thumb move the floss in between all your teeth one by one. When flossing, make sure you are not cutting your gum. The goal is to clean the teeth surfaces and not the gum. In each space in between the teeth, press the floss against each tooth (hug the tooth) and gently move it back and forth and up and down and then move to the opposite surface of the adjacent tooth.