Not much to say on this one, nice and easy.
Only way to get RGB out of a N64 is to get hold of a US NTSC console with a VDC-NUS video output chip - typically the earlier models.
Just 3 wires to solder from the board to the N64 AV out port, allowing you to use your SNES RGB cable too....
All the pics and info you ever needed can be seen on mmmmonkeys great website here:
No point me repeating this - it's done much better that I could do it.