rear seats bolt straight up but you need to fabricate a mount for the base of the upright as the Mk3 seats are 2 part whereas the mk2 are 1 piece. Also the bolts at both top corners (under the parcel shelf supports) if you wind them out a couple of turns it makes it easier to fit, as the mk3 latches are slightly different.
As for the fronts its a bit more completed as the subframes on the mk2 are nearly 10mm narrower than the mk3.
If you (or know someone who) can weld its not a bad job. You will need to cut the legs of your mk2 subframes so either be prepared for it or get some from a scrapped car.
The passenger side is really straight forward, as its a case of cutting the "dogleg" shaped mount off the mk3 and replacing it with the straight mk2.
The drivers side is a little more difficult as the mechanism for the seat height adjustment is there. What I did was to heat the leg up so that it could be bent into the correct position (IIRC this is about 6mm narrower) and then fabricate a bracket and weld that on for support.
If you wish I can grab some photos later this afternoon and post them for you??
Also I have been told if its a 90spec Mk2 then you can remove the lower seat base and remove the cover and foam and replace it with the mk3 ones, although you will need to cut off the bolster bars and replace them with the mk3 ones, as they are slightly larger.
