After Erin asked about a pattern for the car seat, I thought I'd explain how I actually made it.
This isn't a difficult sewing project, but you should have some experience with shaping and an understanding of how things fit together. I wouldn't try this if this is a first sewing project or you're not comfortable with spatial relationships. I didn't have a pattern, but I did have the cover. This is kind of a "point of no return" project. I looked at the cover and figured out that it was three pieces: a top section, a bottom, and a foot piece that wraps around the actual seat. I took it apart at the joining seams. Once I took it apart, I laid it out just as it had been sewn together and this is what it looked like:
You can see how the pieces look like they curve opposite of each other. This allows the cover to curve around the seat. You can either cut the shapes out of newspaper to make a pattern, or just place the cover on the new fabric and trace. You'll need to figure extra for seam allowances. In my case, I had to figure even more since the cover had shrunk in the wash. The hood was the same way; using it as a pattern to cut another.
I used one yard of fabric for the seat, half a yard of a matching fabric for the hood, and one yard of a backing fabric (like cotton duck.) I also purchased some 1/4" batting, and ended up deciding to make it three-ply. You shouldn't over-stuff the cover as that can be dangerous to the functionality. Make sure you launder your fabrics before starting.
After you've cut the pieces out (one each of the front fabric and the backing, three of the batting.) work on the cover in those portions. It's best to finish each section separately and then join them all together. Make sure you're using your seat as a guide to how the cover will fit, trimming and tailoring as necessary.
First, sew the fabric, right side out, with the batting and the backing together. I used a 1/4" seam. Then, using the seat as your guide, lay the top section into the seat and mark where the creases are. I sewed two lines on either side, through the batting and backing, to allow the section to fit in a tailored fashion. You should be able to see this method just by looking at the original seat cover. You'll then mark the cover with where the straps will go. I used my button-hole settings to create the slits.
Repeat this for the top and bottom sections.
The foot piece is simply lined with the backing, no batting, before it's attached. Pin and sew all the sections together.
The hood is simple once you look at the construction of the original. There is an elastic at the back to catch on the seat. I sewed a tube by folding the fabric in the middle once. That created a space for the small rib to enter. I just used the plastic rib from the original hood. You'll also need to make a folded over section for the main hood support. Really, it's not hard to do, you'll just need to see how the original hood worked.
Now you can trim the outside edges as you desire. In my example, I used my secondary fabric. I cut out 2.5" width strips in lengths. I had measured around the seat and knew how long the strips would have to be. I sewed the strips together and folded both edges to the middle to create a finished trim. I used a flexible iron-on interfacing on the inside of the strips, but that's not necessary.
After the three pieces from the cover are sewn together, you can attach the trim.
Once you've trimmed the cover, you should take the plastic tabs from the original and attach them in the right places on the back of the new cover. I had to sew the tabs on by hand since my machines isn't exactly industrial.
Your cover should fit snugly onto the seat.