1. The harness should be tight
I asked one of the product designers for one of our car seat brands 'how tight exactly?' his response: 'In Norway, we say until they're blue'. OK that's a bit extreme, but you get what he means, right? You need the straps to be firm so that they hold the child in the seat properly, because unlike the adult seat belts in your car, child car seat harnesses don't lock in the event of an accident, so they need to be firm right from the get-go. A good rule of thumb is that you should only be able to fit one finger between the child's chest and the harness.