First, it's important to find a flosser that's comfortable for you and suited to your specific needs. All you have to do is insert it between two teeth and then gently slide it into the space between the gum and the tooth, then rub it well against the tooth.
This movement must be repeated twice for each interstice:
1 - take about 50 cm of dental floss and wrap the ends around the middle finger of each hand. With the thumb and forefinger of each hand, grasp a piece of about 2.5 to 5 cm then insert it, using the thumb, between the upper teeth
2 - while keeping a segment of dental floss (2.5 to 5 cm) tight between your fingers, use your index fingers to insert it between the contact surfaces of two lower teeth
3 - gently move the floss between the teeth in a zig-zag motion. Don't push the floss into your gums. This can cause damage to them. Thoroughly floss the entire tooth. Using an up and down motion, slide the floss against the surface of the teeth and below the gumline.