To install a new seat belt, find the anchor that attaches the seat belt to the car frame. Set a crescent wrench to hold the anchor bolt inside the car and prevent it from turning. Unscrew the anchor bolts from the other side of the frame, and remove them. Take the old seat belt out, and place the new one in its place.
When removing a retractable seat belt, take out the case with the spool that contained the old belt. To finish the job, place the new spool case in place of the old one.
If you are installing a straight belt, find the 45-degree angled anchor plate that comes with it. Install the anchor plate. Put the straight belt through the anchor plate, and secure it around the locking plate. Thread the seat belt through a hole in the center of the locking plate, and wrap it around the top. Pull it through the center of the plate again. At this point, the belt shouldn't move when you pull on it.
When setting up a crescent wrench to hold the anchor bolt, make sure to place the wrench in a way that it is going to get blocked against the car seat if it turns. You can also ask somebody hold it in place while you work with the anchor bolt on the other side of the frame.