This is a very good link.(the good part starts at #18) There are two things that I would like to add. Ply-grip, (or flex-grip as it is also called) comes in two different styles. There is a heavy duty style and a regular duty style. Use the heavy duty style. (any upholstery shop should be able to get it for you,........ at least my shop would)
Also, you don't need to cut strips of cardboard. Any upholstery shop can sell you 1/2" wide strips of cardboard (called blind tacking strip) which comes in 38" strips or in roll lengths.
The main thing to remember is that you need a solid base to attach the plywood base to. If your car does not have metal braces to screw the plywood to, you will have to fabricate your own, and that's when things get tricky.