Ok, here is a picture of the seat you have
Originally Posted by tinyblackdot
Wait wait, I am totally confused. I went and looked in my DHs car, and the seat only has 1 slot for the straps. What thing that slides up and down?
It only has one "slot" and that slot slides up and down, clicking into different settings for the harness. See those red tabs on either side of the headrest? You squeeze those together, and slide the headrest up to change the harness height. If you slide it up all the way, that's the setting that's for booster mode only. Click it into the next-highest setting, and when the kiddo's shoulders reach that level, the seat is outgrown.