All the mounting holes are already there for rear seatbelts so fitting them is relativly straight forward, any panals that require cutting (piller trim and parcel shelf mount) are all rarked and can be cut with a jigsaw.
Make sure all the required bolts and washers are taken out of the scrapped car with the belts and make sure the are properly tightened when put in. This is made difficult as there is probably some rust or paint in the bolt holes so a lot of force is required to tighten the bolts, so i would use a vcery large socket set wrench to do this job.

The order of the seatbelt ancourage points, belt catches and lap belt go in are as such :-

From Drivers side, Drivers side rear belt anchorage point, Lap Belt, Drivers side rear belt catch, passenger side rear belt catch, Lap Belt Catch, passenger side rear belt anchorage point.
This may apear odd but this is correct and holds rear passengers tightly in the seats in the event of a crash.

For the seat belt reels, make sure they are tight and slightly angled forward (they will only go in like this using the supplied mounting plate). This makes sure that the belts slide in and out smoothly and do not stick.

Once instalation is completed make sure all anchorage points are as tight as possible and that the seat belts operate correctly, if you are unsure if they are correctly, or adequatly fitted, check with a qualified mechanic or take the car to an M.O.T. centre and make sure they are safe.