Refillable floss is measured for your mouth, with pre-marked sections about 18” long. Just pull the string until you see a green marker, trim, and start flossing.
Dentists recommend using about 18 inches of floss at least once a day. This gives you enough string to use a fresh section between each tooth, and avoids spreading gunk around. You should always floss with a little bit of slack to maintain control, and gently wiggle the string down to the gumline following the natural c-shaped curve of your teeth. Not sure if you should floss or brush first? Truthfully, there’s no wrong choice. Just be sure you floss, and floss well.