Ask the retailer to help you familiarise yourself with how the seat fits and works, and check the instructions carefully (if you lose them, you may be able to get them from the manufacturers website).
When fitting your child in the seat, remove thick clothes because these reduce the effectiveness of the straps. Once the straps are tightened, you should be able to get just one or two fingers between the childs chest and the harness. Periodically, check the harness is still adjusted to the right height, according to the manufacturers instructions, and keep an eye on adjustable harness webbing, which can work lose from its mounts.
For booster seats and cushions, make sure your cars seatbelt isnt twisted, and that the lap portion lies over your childs lap, not his stomach. The shoulder strap shouldnt cut into his neck, either.