If your car only has lap belts, you can use them to secure a forward-facing car seat for a shoulder belt with a built-in harness. For booster seats, you might need to consider a lap belt-only approved booster or have shoulder belts installed in your vehicle. Some car seats also come with accessories or kits allowing lap belt-only installation. Always consult the car seat manual, vehicle owner's manual, and vehicle installation features. Consider professional installation assistance if needed.