Get a wrench and screwdriver.
Find the bolts that hold both ends of the seat belt in place. You may need to pull a plastic cover off the top of the bolts to see them. The cover may just pop off or you may need to pry it off with a screwdriver. Some cars have these covers, others don’t. The bolts may be attached to the side of the seat, the floor or the side panel. If you have a three-stage seat belt, you will have the shoulder strap, lap belt and the buckle to unbolt. If you have only a lap belt, you will only need to unbolt the belt and buckle.
Use your wrench and unbolt them all. Check that you unbolt the retractor side as well as the side the seat belt locks into.
Then you can pack it and securely place the order online for your seat belt repair.