Move the front seats up as far as possible so that you have lots of room to work in the back seat.
If possible, recline the vehicle seat where the car seat will go before starting your installation. Make the seat upright once you’ve finished. This will help you get the seat installed tightly.
Pull the seat belt or latch belt from inside the car seat, not from outside the seat. This means that if you dropped the latch belt, it should fall down on top of where the child goes, not outside the car seat. This rule applies to most car seats except for the following:
Britax Classic convertible models, older Britax convertibles (i.e. Marathon, Boulevard, Roundabout, Decathlon–those that go up to 40 or 65 lbs)
Chicco KeyFit and KeyFit30
Sunshine Kids Radian models
For Infant Seats specifically:
Put your weight on the base when you pull the seat belt or latch belt tight. Do not just use a hand on the base, this generally won’t be enough weight to get the seat installed tightly. In this photo, Alisa has her left knee in the base and her right foot on the ground. If you are short and flexible, like all three Car Seat Ladies, you can probably get your foot in the base–this works even better.
Put weight on the right side of the seat, and then the left side, then the right side again, etc, as you pull the latch belt or seat belt tight. This will help you get the belt as tight as possible. In the photo, see how Alisa is pushing with her hand and her knee on the right side of the base. After she pulls the latch strap, she’ll move her knee into the left side of the base and push there as she pulls more slack from the latch strap.
Unless you are installing a Chicco Keyfit, pull the latch strap or the seat belt from inside the base, as Alisa is doing in the photo.
For Convertible Seats specifically:
To tighten the seat belt or latch belt, go behind the car seat, between the car seat and the front seat. Use your stomach to push the car seat into the back of the car and sway from side to side as you pull the seat belt or latch belt tight. You will have to lean over onto the car seat. Note how Alisa is leaning over onto the car seat and pushing it with her belly as she pulls the lower latch straps tight. Also note that the vehicle seat is very reclined. She will move the vehicle seat upright after she has installed the car seat.
If you don’t fit behind your rear-facing convertible seat or if you don’t want to use your belly to push the car seat into the vehicle seat, you can stand or kneel beside the car seat and push down with one hand on the seat (put your hand where your baby’s bottom would go) and pull the latch strap with the other hand. Sway the car seat as you do this. Then switch to the other side and repeat. You may have to switch a few times before you get the car seat completely tight.
The following videos illustrate many of the tips above and may help you get a tight, secure installation. Of course you should always read your car seat’s instruction manual before you attempt any installation.