Although it is optimal to use a car seat base with a car seat, there may be times when this is not possible, such as while traveling. You may need to install your car seat in a taxi cab, a friend or grandparent's vehicle or even on an airplane. It should be noted that there are some car seats that cannot be installed without a base -- check your owner's manual to be sure.
Position the car seat on the back seat facing the rear. Infant car seats must always face rear -- never forward or even sideways.
Route the lap belt portion of the seat belt under the belt guides and buckle it.
Make sure that the level line on the car seat, if there is one, is level with the ground. Place a rolled towel underneath the front part of the car seat if needed to keep the seat stable and secure.
Tighten the belt as much as possible by pulling on the shoulder section until it locks.
Push the car seat into vehicle seat and seat back with your arms and tighten the seat belt even further.
Make sure the belt locks into position each time you tighten it. If it does not lock, you may need to purchase a locking clip.
Double check what you have done. Make sure the seat belt is tight and locked, the car seat is stable and cannot be rocked more than one inch back and forth or side to side. If there is a line on the car seat to indicate level, make sure that it is level with the ground.
Local sheriff's offices, police stations or fire stations usually have someone on staff who is certified to check that car seats have been installed correctly. Take yours to be checked if you have any question at all about its installation.
Installing a car seat without a base is not optimal. Using a base is always the best way to ensure the car seat is as secure and stable in the vehicle as possible.