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 1 foot). Wrap both sides of the string around your middle fingers. Using your index and thumb, glide the string in between all your teeth one by one. To floss between your teeth, curve the string into a C-shape and press it tight against one tooth. Then, slide it up and down the tooth and root surface, going just under the gumline. Be careful not to force it further than it wants to go since this action can irritate or cut your gum tissue. Repeat this process on the tooth on the other side of the space.
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.