Could try the glitch thread, but just in case it isn't there, you need to GBs, another Red or Blue, and a link cable. Trade with the other game the Pokemon you want to clone, and a Pokemon you don't mind losing. The game that is getting the Pokemon that you aren't cloning turns off before the trade is complete. I am 90% sure that thatis all there is to cloning in R/B, not if it works with Yellow.
woot21 out dawgs