If you cannot get the belt tight, look for another set of belts in the car that can be tightened properly. Read your car owner's manual to see if your seat belts can be locked into position or if you will need to use a locking clip. Read your instructions for information on how to use the locking clip. Some lap belts need a special, heavy-duty locking clip. Check your car owner's manual for more information.
Some car seats come with a tether strap.
A tether strap is a belt that is attached to the car seat and bolted to the window ledge or the floor of the car. They give extra protection and keep the car seat from being thrown forward in a crash. If your seat requires or recommends a tether strap, be sure to install it according to instructions. The tether strap may help make some seats that are difficult to install fit more tightly. Keep your child in a car seat for as long as possible.