To tighten a seat belt, first fasten it around whatever needs to be restrained, then pull the belt lightly away from the object and release. The seat belt automatically takes up any slack and conforms tightly yet comfortably around the retrained person or object.
Seat belts are designed with retractor mechanisms that automatically pull the belt webbing backward into the car seat when slack is present. The mechanism contains a spring that gets unwound when the webbing is pulled out. The spring then winds itself back up when the pulling force is released. This automatically tightens the belt. If there is still any slack present, the belt can be further tightened by moving the latch to the desired position.