You will have to simply, put the bumpers up to the car, with the brackets mounted to the bumpers, as if you were going to mount them. Keep in mind, that this will be alot easier with two people. Look at the bumpers closely and make sure they are in the correct position. Then, you will have to drill holes, front and rear, for the bumper brackets, to mount them to the car. The big bumper cars don't have provisions for the small bumper bracketry.
O.K. Now I understand. Do a search on this message board. There is a technical article detailing exactly what all you need to do. For example: http://mmb.maverick.to/showthread.php?t=44352 http://mmb.maverick.to/showthread.php?t=56879
Push in hard "reward" on the base of the seat cushion and lift the cushion out. Once u get the cushion out u will see the 2 screws holding the seat back in place - remove them if u want to take the seatback out.