How can I make the car safe for my 6-month-old?

A seat belt car seat. Make sure the baby's size and weight are appropriate for the car seat and seat belt adjustment!
Rear-facing car seat. Your baby should be in a rear-facing car seat until s/he is at least 1 year old and weighs 20 pounds.  make sure your child still meets the height and weight limits of your rear-facing car seat.  carefully follow the manufacturer's instructions when installing new or converting old car seats for your child.