As you are going to put your fingers into your mouth, be sure that your hands are clean.
Cut a sufficient length of dental floss, about 45 cm. It may appear too much, but it is necessary to keep a clean part of the dental floss while you pass it between two teeth.
Wind up the greater part of the floss around the forefinger of one hand and the smaller one around the forefinger of the other hand.
Slide gently the dental floss between two teeth, passing from one tooth to another, and being careful not to break the floss between the teeth.
Pull the dental floss up gently, from the edge of the gum to the upper part of the tooth.
While passing from one tooth to another, unwind a clean segment of the dental floss starting from the forefinger of one hand, while winding the used part of the floss on the forefinger of the other hand.
Use your thumb to guide you.
You can find accessories like “dental floss holder” in order to help you in case you cannot manage in the traditional way.