Car Seat help!

Ash • Married 8.2.2014. 1st Boy 11.24.16. 2nd Boy 8.27.18
When it comes to car seats, I have no idea what I'm doing. I got one of those head bumper things and I put it in the seat and it looks like it'll be too tight. Then with a snow suit on, because it'll be November in New York, I feel like it will for sure be too tight. Should I loosen out the straps? I have no idea how to do this car seat deal. Lol