For many years, a locking clip was used to lock all lap and shoulder belts when installing a car seat. This was the only available method for locking the belt. Today, there are a number of other options available.
Everything you purchase comes with a manual, whether it is a lawn mower or a DVD player. When you are preparing for a child, multiply that by a bouncer, a high chair, a stroller and a swing. Chances are, you read them to know how to put them together and their basic function.
Before installing a car seat, the most important step is reading the manuals for both the vehicle and the car seat. The vehicle manual will tell you what kind of seat belts your vehicle has. The car seat manual will tell you what kinds of seat belts the car seat can be used with and if the car seat has any additional locking features of its own.