Pretty self-explanitory. Just progress through the Uchiha chapter, battling Itachi with Sasuke a few different times. You'll also have to fight Kisame using Suigetsu, which can prove to be difficult. Be prepared. Try blocking or entirely avoiding Kisame's ultimate jutsu. Itachi isn't too hard of a fight, especially with a list of quicktime button sequences. Use you charged Fire Dragon jutsu to maximize the gap.