Here's a few general rules for properly buckling up baby:
#1: Make sure that the harness lies flat on baby, with no twists in the straps.
#2: For a rear-facing child, the straps need to be at or below the child's shoulder.
#3: Tighten up those straps! It can be pretty daunting tightening up the straps on a brand-new baby, but keep in mind that if the car were to suddenly be upside down, those straps need to hold baby safely in the seat, and in order to do that, they need to be tight. Make sure you pull all the slack from around the baby's thighs, and pull the harness adjuster gently to tighten. The straps need to pass the "pinch test": try to pinch a horizontal fold in the strap where it crosses the baby's collarbone. Your fingers should just slip off; if you can pinch any fold, you need to snug up the harness some more.
#4: Position the chest clip properly. The chest clip needs to lie...well, on the baby's chest! ;) The perfect spot is with the top of the chest clip right in line with baby's armpits. The sternum is the strongest point on the infant's torso, and having the chest clip low on the belly can cause damage to abdominal organs in a crash.
#5: Make sure the infant seat is at the correct angle. Newborns have no head or neck control, and it's imperative that their car seats are reclined at the proper 45* angle. If the infant seat is too upright, it can cause the baby's head to flop forward, with the baby's chin on her chest, cutting off the airway.
#6. Use only what came in the box with the car seat--no aftermarket products. Those super-fluffy infant positioners, strap covers, head positioners and 'bundle bags' are all tempting, but they are not considered safe for use with car seats. The general rule is nothing between the car seat and the baby, or the baby and the straps. Any additional bulk will compress in a crash, introducing slack into the harness and putting the baby at risk to be ejected from the seat.
Here is my third baby, correctly buckled and ready to go!