As with all child car restraints, follow the manufacturer's recommendations on how to fit and use them. Depending on the type of seat, it will be fitted with the car's seat belts or if it is an Isofix or i-size seat, it will fit into the Isofix anchorage points that are built into the car.
Seat fitted with the car's seat belts
Forward-facing seats that are fitted with the car's seat belts have red guides through which the seat belt must be threaded to secure the seat. The seat belt is threaded through the red guides on the child seat. The correct way to fit it will be shown on labels on the side of the seat.
Isofix forward-facing seats
Forward-facing Isofix seats do not use the car's seat belts. They have mounts built into them that slot into the Isofix anchorage points that are built into the car. They will also have either a support foot that needs to be extended to the floor of the car or a top tether that fixes to the top tether points behind the car seat.
Check whether your Isofix child seat is approved for your car by contacting your car manufacturer and/or the child seat manufacturer; many have a list on their website that shows which seats are approved for which cars.