Summarize this article:

To provide the most protection, newborns should always be in a rear facing seat. Until they're at least a year old and potentially up to age three, a rear facing seat is an option. Move up to a forward facing seat for older infants and toddlers.  How long to keep your child in a rear facing seat will depend on its height and weight, as well as the manufacturer’s specifications. Experts recommended keeping your baby on a rear facing seat as long as possible. Use one until your child reaches the upper height or weight limit set by the manufacturer. Some car seats are all-in-one, starting out in the rear facing position and converting to forward facing when your child is available. Check the owner’s manual for  instructions on how to convert your seat. Child car seats should always be positioned in the backseat, never the front seat. Many drivers like to set them behind the drivers seat to make it easy to check on them from the rear view mirror, without turning their head. Once installed, the car seat base should not move more than 1 inch (2.5 cm) in any direction. Whichever kind of car seat you are using, always pull on it a bit in all directions before placing your child inside. If it seems to move too much, tighten any straps or belts that are too loose. After placing your child in the car seat, place the two straps over their shoulders and buckle them at the lap harness. Buckle the chest clip and make sure it stays at armpit level. Tighten the straps until you cannot pinch any extra material on them at the child’s shoulders.  Make sure the straps are not twisted. The correct way to tighten the straps will depend on your model of car seat. Most have a place to pull on and latch the strap. Check the manufacturer’s instructions if you aren’t sure. For safety reasons, do not place extra material (such as a blanket) underneath the straps. If you want to cover your child, buckle them in first, then add the cover.

Summary:
Select a car seat based on your child’s age, weight, and height. Pick a spot in the backseat. Check the seat's fit. Secure your child in the seat.