What is the “best” car seat for my child?

Is it OK to use a second-hand seat?

I’ve had this seat for a while. Can I still use it?

There are lots of after-market items for car seats that I can buy. Are they OK to use?

My child’s car seat has a five-point harness. Where exactly do I place the harness clip?

How snug should the harness be on my child?

How high should the shoulder straps be?