You'll need to remove the rear seats if there are any and then bolt the harness's mounting point onto the rear part of the chassis. The belt should be at no more than a 45-degree angle from the shoulder line. The process varies depending on your car's make and model, and the type of seats that you have in the car. It's best to follow the instructions religiously, most renowned harness makers such as TAKATA provide you with a manual. If you're unsure, best to have a professional install it for you. Racing harnesses are essential for safety and incorrect installation can cause more harm than good.