It is much easier to understand how to install everything, when you have the seat and instructions in hand. Don't worry you will be able to put it in, and adjust everything.
I bought the small car seat for Gracie. She Loves it, I wish I had bought the bigger one because I have 4 Yorkies and they all want the car seat. I did buy a larger second car seat but 3 will not share, so someone is always stuck in a crate in the back.
My SUV has a doggy gate behind the back seats, so the whole rear is really a giant crate. But I just don't feel like they are safe running around free back there. (I do let our Lab ride back there when she is with us.) On long highway trips I prefer my Yorkies to travel most of the time in their crates. In the gated back hatch area. I feel like this is the safest place for them in the event of an accident.