Measure car seat and work out what kind of pockets you want
Measure up with at least 1 cm extra on all sides for hemming
Check those measurements again
Flip top over 10 cm and pin right sides together
Mark 10 cm in from top and on each side
You're going to cut the corners off
Turn right side up
Fold over sides and iron, ready to sew seams on each side top to bottom
Pin and sew
Now for pockets... Cut strips longer than your width, and add facing to top of each. These will be the top of your pockets.
Lay them out and plan where they need to go
Bottom pocket... Fold over sides as shown to give some extra space in the pocket
Sew elastic just under facing to help keep pocket tip tight
Pin top pocket, allowing "bulge" space to accommodate water bottles or pencil cases
Sew in bottom pocket
Add goodies... And Voila! Hers!
Don't forget buttonholes to attach to top of car seat, or straps if that's easier
And elastic sewn to bottom corner of organizer... You'll pull this around the front of the seat and pin to other corner of your organizer to pull it taut.