I did make two of these car seat covers which are very popular and expensive. The first one is made out of two layers of flannel. I measured the car seat and used those measurements. I also made one for a gift and used the self binding pattern for baby blankets as the cover and attached the handles.
When it got colder, I purchased two warm baby blankets, applique on the front, sewed together and added handles after lining up with car seat. Purchasing the blankets was cheaper than buying the fabric.