How Too....
Needless to say the seat has to be taken out of the car....
Remove the two screws under the seat and unhook the latches on the back rest and take the seat out of he car...
I found it easier to separate the bottom from the back, this is done by undoing these two screws....

Next undo the back rest rubber pull release with a flat screwdriver...

Then remove the grommet thingy that the rubber pull release units plug into... I used a 3/4" block of wood and a hammer and knocked it out from where the latches are located between the back and the metal frame, they do have a couple of securing pegs that may hamper you just pulling them out as I found out...

Now undo the seat cover around the metal frame ... this simply pulls out... I started at the bottom where one of the wire supports joins the metal frame.. You could use a flat screwdriver but take care not to rip the seat cover... If you take your time and just use your fingers it will work - cost me two broken finger nails.

Now you can CAREFULLY remove the seat cover to free up the cardboard back and slot in your new carpet backing... Note the backing has 6 locating lugs in the metal framework use these to hold the carpet and cardboard in place when reassembling...
Reassemble in reverse...
The only thing to note is when you are putting back the seat cover to the metal frame is to DOUBLE Wrap or fold it over TWICE, the Plastic edging which is stitched to the seat cover.. This will make the seat cover nice and taut... again use your hands as to avoid ripping the fabric and it will CLICK into place in the metal frame...

Job Done...

Hope you find this useful..