Rear Facing Installation
That is the common installation process for a convertible carseat in rear-facing position. It is important that the shoulder/lap seatbelt be as tight as possible. This can best be achieved by putting as much of your own weight as possible on the carseat (without the baby of course), clicking into the seatbelt fastener and then pulling as much slack as possible out of the seatbelt strap. If you have the assistance of another adult, one should provide the weight on the carseat from inside the vehicle while the other pulls on the seatbelt strap from outside to remove as much slack as possible. If you have an I-shaped locking bracket, it should be installed over both straps 1 inch from the seatbelt fastener.
Some, but not all, manufacturers provide a metal locking bracket to be placed on the seat belt after the carseat is installed to prevent excessive slippage. You could contact your manufacturer to see if one should have been provided or you may purchase one at most major baby product retailers.
Be aware, a little movement of the installed carseat is considered normal and acceptable.
One last note, many fire stations, police stations, children's hospitals and even car dealerships offer free carseat safety checks. Call those in your area and ask if they offer this service.