Here's what you need:
A cheapo/uggo seat cover that you'd never wanna use in a million billion trillion years.
A sewing machine
Around 1.5 yards of 1/2 inch wide elastic
Something to copy your pattern onto (newspaper, carboard, posterboard)
Music (to have a mid car seat making dance party or just enjoyment.)
**For fabric, these are my suggestions:
For this use the fabric of your choice, but I will admit that for the seat part of your cover (where you sit and where you lean back) Cotton, twills and normal woven fabrics will work better. For the back and the underseat of your cover, you should use a knit or fitting it to your seat might be difficult.
You will need approximately 1.75 yards for the seat part. 1.75 yards for the back and underside of the seat, and 1.75 yards of the quilt batting for the comfort of your seat. (These are high estimates, you can probably get away with 1.5 yards, but you will have to squeeze it in tight.)
you can view it here http://www.etsy.com/view_listing.php?listing_id=6328532
Step 1: Find the ugliest/cheapest universal carseat cover.
So, you don't really need a seat cover to de-construct, you can look at my pattern pieces and copy them to some extent, but to make sure it fits great, you really should get a seat cover you can destroy first. I've been told you can get these at salvos (salvation army's) or other thrift stores.
**please note** You can also measure your own seat to make the covers specific to your own car. I still suggest making what I'd call a "princess" seam (because it helps the seat fit nicely. It also give a better, more finished appearance.