A good carseat installation means that the seat (or infant seat base) moves less than 1" when you tug on it at the belt path. Don't try to shake the rivets loose on the carseat—just tug.
Use this section to get your installation questions answered. Are you a visual person? There are videos and pictures to help you. LATCH has created more problems than it has solved, so visit the FAQ and Installing Your Carseat pages for more help with that topic.