Depends on the model.
Early NTSC models that have the VDC-NUS encoder, very easy. THS7314 amp and some resistors.
http://www.mmmonkey.co.uk/ntsc-nintendo-64-rgb/
Other than that, you just have to decide how your scart cable will be wired for sync (luma, csync or composite). NUS-CPU-04 motherboard revision still has the VDC-NUS encoder, but needs csync to be rebuilt if you want to use that. The difference between where sync is taken from, is just an arguement over picture quality and a "hatching" effect that can show up on some enquipment. Generally for a pal tv with native rgb, composite sync is fine, while luma or csync seems prefered for scalers. Some tvs can show the hatching effect though, so I would probably go with sync from luma or csync. If you've already got an rgb scart cable, unless specified otherwise, it's probably taking sync from composite, so you can always just try it, if no hatching stick with it, if not, get a cable that uses luma or csync.
Anyway, back to the actual mod.
If you don't have a revision with VDC-NUS encoder, then it's a little more effort and expense, but can still be done. It's dac replacement time
You will need this
http://etim.net.au/shop/shop.php?crn=209
and instructions
http://etim.net.au/n64rgb/instructions-new/
There is an earlier revision of that (still works fine), that can be programmed and built yourself.
http://members.optusnet.com.au/eviltim/n64rgb/n64rgb.html
If you don't want to open and check, this guide here will give you an idea of what encoder you
http://forums.benheck.com/viewtopic.php?f=5&t=27684
Ignore any info that says "only X models can be done". All can be done with the replacement dac method, just a lot of pages are out of date or just ignore the replacement dac method because "it's too hard" or other nonsense.