Now that you have a seat and it's safely installed (right?), the next step is making sure you're using the car seat correctly. After all, even the most expensive car seat, installed correctly, doesn't work at all if the baby isn't strapped in right.
So let's get started!

Rule #1: Make Sure Shoulder Straps Are in Proper Position
When baby is rear-facing, the top of the shoulder straps have to be at or below the baby's shoulders. When forward-facing (which your baby shouldn't be), they need to be at or above. This is measured perpendicular (at a right angle) from the recline of the back of the seat, not from the ground. When in doubt, put a popsicle stick into the slot with your baby in the seat to check -- sometimes the fabric of the seat can make it hard to tell where the strap really is.

Rule #2: Rear-Facing Is the Responsible Choice
Unless your child has serious medical problems, they have to rear-face until they literally cannot anymore. With all the new, cheaper seats with 40-pound or higher rear-facing limits, it's possible for anyone to have a seat that will keep their child rear-facing until the bare minimum of 2 years old and 30 pounds, as per the AAP's improved guidelines. But as they state, 2 years is the bare minimum -- your child is still significantly safer rear-facing until you cannot fit them that way any longer. After all, it's 500 percent safer.
Children's heads are a significantly larger portion of their body than an adult's and their spines are initially cartilage -- not bone. In the event of an accident, if their overly large head pulls on the weak spine (which doesn't fuse and harden until around 4 years old), they will likely suffer from internal decapitation -- this means that the cartilage and spinal cord inside their neck snap and they die instantly. It takes a serious medical problem to make that risk worth forward-facing a toddler before you absolutely have to. (I don't want to scare anyone -- this is all to help us keep our kids as safe as we can.)

Rule #3: No Gaps Allowed Between Baby's Crotch/Groin Area and Harness
This is really only an issue with newborns, but an important one. If there is a gap between crotch and harness buckle, roll up a washcloth or receiving blanket and put it in a upside-down U shape, with the middle between baby's crotch and the harness and the rest lying flat between the legs. This is one of the only "add-ons" allowed. When in doubt, call the manufacturer.

Rule #4: Be Sure the Chest Clip Is Positioned Properly on the Chest
The chest clip belongs between the nipples and armpits. This positions the straps so your child doesn't fly out of the seat -- and can cause damage when placed anywhere else.

Rule #5: Know the Proper Guidelines for Outgrowing a Seat
Outgrowing a seat has nothing to do with legs touching the seat. There has never been a case of legs breaking from touching the seat and even if there were -- would you choose for your child to break their legs or their neck? Only one of those can be fixed. Your child has outgrown a seat in weight when they reach the max limit for that position. This is non-negotiable. When rear-facing, your child has outgrown their seat in height when there is less than an inch of the hard shell left at the top of the seat above your child's head. This is measured perpendicular to the seat's recline. However, there is a new seat that has different guidelines -- so be sure to read the manual for your seat!


See how the strap can be pinched?
That means it's too loose.Rule #6: Straps Need to Pass "The Pinch Test"
The old rule of "two fingers under the chest clip" is outdated and resulted in straps that were way too loose. When straps are too loose, the child can fly out of the seat, or get stuck halfway and break god-knows-what.
The new rule is to first pull any slack tight from the lap-portion, and then pinch the straps at the collar bone (your fingers are pinching top-to-bottom as shown here). If you're able to pinch the strap, it's too loose.

Rule #7: Coats Are Not Allowed
A cop reports seeing a coat strapped into a seat, even after the child who had been wearing it flew out of it. Coats are not safe in car seats and almost all manufacturers have this rule in their manual as well. To understand why, place your child in a coat and put them in their seat and tighten the straps properly. Without loosening the straps, remove your child and remove their coat. Now place them back in the seat. That is how much room your child would have once the coat compressed under pressure, much like how you can squish a pillow if you sit on it. If there is more than a tiny bit of extra slack (like the difference between sweatpants and stretch pants), the coat is too bulky. Instead, try taking your child's coat off right before putting them in the seat, buckling them in quickly, and then putting their coat back on them backward. I also keep blankets in my car for safety in case I'm stranded, as well as for the kids to use in winter. I even just bought two kid snuggies for both of my children to use while in the car, or you can buy a car seat poncho.

Rule #8: RTFM (Read the #^!&ing Manual)
Most car seats have a specific location for the manual on the seat so that it's kept with it at all times. That's because almost every single question you could have is listed in that little booklet, and the manufacturer's number is there for anything else. Use it. Even car seat pros utilize this booklet. Every single car seat is different with different rules, and this booklet (or the PDF of it online if you lost yours) is your golden ticket to proper car seat usage.

Rule #9: When in Doubt, Get Help
Skip the fire stations and police department -- they mean well, but often are no more trained than you are. Contact SafeKids at their website or at 1-866-SEAT-CHECK.
There is no shame in asking for help, but there is shame in letting your ego or embarrassment stop you from making your baby as safe as they possibly can be.

Images (top to bottom) Christie Haskell; Defrost/carseat.org