Erm well, the bottom mounts should be easy, hopefully, you should be able to mount them onto seat mount points, the rear points are more difficult, you may be better off getting a 3 point harness and taking the back point to the bolt point for that bar that goes across the car inbetween the back seats and the boot - a possible anyway, but as Ive not tried to do this I maybe completely wrong.
Remember that the law states that harnesses/seatbelts must be permanently fitted to the vehicle, so dont use the clip style if you are going to get rid of the normal seat belts, if youre gonna keep them then either is okay (apparently)