You don't secure the back (if by the back you mean the part closest to the front of the car). The weight of the baby holds it down once clicked in. When you click the the carseat into place (you will probably hear a click as well) it should not lift up at all and stays in the base securely. If you're unsure if you've installed properly (as these things can cometimes be tricky) you can always take it to the local police dept. where they will show anyone free how to do it and if you've installed it right.

I second the above reply. Take it to your police/fire dept. and have it inspected in your car to make sure it is properly installed. They all do free inspection days.

The manual should explain how to secure it.we use the extra belt and the anchor in our car and it is secured nicely