I find that with so many things, the little added touches...the details, really do make a difference!
In order to make the new carseat cover removable (and washable), I had to cut slits to allow for the seatbelt straps. To keep the edges from coming apart I sewed around where the slits would be cut. I also added buttonholes down the sides.
I made some "laces" out of polka dot fabric to keep the carseat cover sides together. I thought bows tied at the bottom would be a cute little detail.
Another detail I thought would be nice to have was seatbelt strap covers. I found this tutorial that suggested using potholders. Since I wanted everything to match, I traced the potholder onto my quilt fabric, cut it out, and added double folded bias tape and velcro.