My fabric choice is probably not very popular, but I just grabbed the least girly prints out of the remnant box at Joann. I used homedecor weight fabric for this project. I was very surprised how my just regular sewing machine handled the fabric. In some instances it went through 8 (!) layers of this fabric without a hitch. (Very impressive). To make it sturdier and less floppy I doubled the fabric for the pocket panels as well as for the main part of the organizer.
Here is how I cut my fabric:
Main Panels: 2 pieces 16x24 inches
Pocket panels: 4 pieces 9x16 inches
Pencil pocket: 1 piece 6x6 inches
Headphone loop: 1 piece 10x3 inches
Ties: 4 pieces 13x2 inches (or 2 pieces 7x3 inches which you can sew into loops if your headrests are removable)
Elastic: 18 inches of 3/4" elastic
I assembled all the pockets and loops/ties/elastic to the front panel first, before sewing the back panel onto the front (right sides together, leave a small opening at the top between the ties for turning). That way you do not have to worry about finishing the raw edges on the panels, except for the bottom of the top one. Then I turned the organizer right side out, pressed and topstiched all the way around. As a side note: to double the pocket panels I stitched two of the pocket panels along the longer side ride sides together, turned, pressed and top stitched.
You should measure the back of your carseat before cutting. My measurements may be a little small for other cars, since my son sits in the third row and the organizer will go onto a second row seat.
After I was done I discovered a great tutorial at TheMayFly. Check it out. It looks very easy to follow.