First, you need to make sure that your cover is big enough to cover the car seat. I have a massive Graco car seat, so what worked for me was cutting my top and bottom fabric to 35"x 42" dimensions [which meant adding a border to the quilt top that I was converting into the top layer.] There are so many tutorials that give instructions on how to make them, but this one is what I followed. The only difference I made from the linked instructions was to use ribbon ties instead of Velcro wraps for securing to the car seat handle. The reason I did this was two fold; 1) I couldn't decide what fabric to use for the wrap; and 2) I really wanted to keep this cute and feminine. The bows seem to add an extra bit of whimsy, and remind me of ballet slippers which is exactly what this cover needed!
If you're looking to make one of these covers take a look at the tutorial and dive right in! I'd love to see a picture of your completed project - leave a link to it in the comments section so I can check it out!