***A note about car seat safety: this type of cover is considered safe and will not void your car seat's warranty in the event of an accident because it does not interfere with the straps or safety features of the seat. There are many car seat buntings that go under the child or that have you thread the straps through them. Those are not safe and if you use them, your child cannot be guaranteed to be safe in the event of a crash. Any extra padding can compress and allow too much slack in the straps (which are made to stretch up to 6 inches in a crash) so that the child could slip out. Anything interfering with the straps could prevent them from functioning correctly or stress them in the wrong way so that they break.***
OK, now that that's out of the way, onward! Just to warn you, this is more of a description of how I did this, rather than a tutorial. I couldn't figure out how to make it less confusing, but I think someone could take my ideas and modify them to make one themselves....
To help you visualize what we are making, all it is is a ring of fabric with a circle sewn onto it. I used about 1.5 yards of 60 inches wide fleece for the cover--or about 3/4 yard of outer fabric and 3/4 yard of lining fabric. I was kind of flying by the seat of my pants on this, so I didn't actually know I had enough until the very end. ;)
First, measure your car seat. Mine is 28 inches tall, 14 inches wide, and about 76 inches around. I played around a bit before figuring out that I preferred my long strip of fabric, which would make the ring that goes around the car seat to hold the cover on, to be about 8 inches wide. I added a bit of length and width and started cutting.
I seriously should have taken pictures of this part, but I didn't...feel free to ask questions in the comments so I can clear anything up...
I cut 4 strips of fabric 40 inches long by 9 inches wide (2 outer fabric, 2 inner fabric). I joined the matching fabrics into into 2 long strips of about 79 inches long and then sewed them all together (right sides together). So now you have your outer and lining fabric stitched into a long tube, right sides together.
Before turning the right side out, I wrapped them around the car seat, pinned it in place, and marked where the handles were so that I could contour the fabric a bit. I pinned along my markings and sewed along the line. Then I cut off the extra fabric and turned the entire tube right side out.
So now you have a shape something like this (pardon the mess, our toddler's play area has the best light):
Next step: fitting the fabric to hug the top of the car seat. I arranged my fabric ring so that the contours lined up with the handles and grabbed some pins. I started folding the fabric at the corners this way and that until I liked how the fabric folded around the seat. I ended up doing just one fold at each corner, but looking back, I think the bottom of the seat (where the baby's feet go) could have used one more fold or pleat right in the middle.
Now it's time for elastic! Find where you left the openings for the channel and cut your elastic. I cut mine at about 50 inches long. Thread it through using a big safety pin or one of those cool threading things that would be soooo nice to have for this project. :)
Now you stitch the top of the cover onto the ring of fabric. This takes a bit of maneuvering since you're fighting elastic, too.
Now that you survived that uber-confusing explanation of how I made this thing, have fun trying it yourself! I recommend having a car seat right there with you while you are making it because there are so many styles and models of car seats. Unless you have the exact one I have, you're going to need to figure out the best measurements for your seat.
If you read all of that, congratulations! I hope you enjoyed it. :)