isn't that hard to do.
1. take of the glass/plastic cover of the original taillights.
2. cut of almost everything but not the mounting bults and frame (after you taken of the black plastic cover)
3. screw apart the skyline taillight.
4. cut of the flash/backlight (it wont fit, so you have to solve the flashing by connecting the lamps so you flash red, or build on a pair of Skyline R32 taillights hence they got the flash light between the brakelights)
5. place the plastic cover for it over the original frame and se where you have to cut it to be able to make it fit, be shore to place the holes in a straight line so you wont get un-symetrical taillights.
6. mount the hole thing back (lightpart and cover part of the R33 taillight) toghether and make final adjustments so it will fit nicely.
7. take the R33 taillight apart again and mask the lenses so they wont get polyesterplastic on them and be destroyed.
8. sand the covers so the plastic will get a better grip (80-grid paper or so)
9. start to use the polyesterplastic and glassfiber...
10. do this a few layers until you're satisfyed. then start to sand and fill up the holes (or how to say it) until you got a surface that can be painted.
11. paint the new modded taillights. i choosed a black color.
finnished result.
good luck!