it's very easy to do
the material at the back of the seat where it folds into the metal seat back is double folded in with a metal clip to retain it in place
just use a small screw driver and VERY CAREFULLY prise the material out of the metal groove
hope this quick pic explains it better
there are loads and loads of these little clips all around the frame so don't lose them and they are bloody sharp!!!!!
you should be able to strip it down so that you can get to the bolts that hold the seat belt mechanism in place fairly easily if you take your time