In some fanfictions, it could be through a special jutsu or ritual. Maybe Itachi, before his death, transferred his Sharingan to Naruto in a unique way, perhaps using a forbidden technique that was known only to a few in the Uchiha clan.
One way could be through a powerful jutsu gone wrong. Maybe a jutsu that was meant to transfer Itachi's memories to Naruto accidentally transferred his Sharingan as well. In the world of fanfiction, there are often many creative and out - of - the - box explanations.
Naruto would gain the ability to copy jutsu. With the Sharingan, he could watch an opponent's movements closely and replicate their techniques. This would be a huge advantage for him in battles.
In some fan theories, it could be due to a unique chakra interaction. Maybe the Kyuubi's powerful chakra somehow mutates Naruto's own ocular abilities and triggers the emergence of the Sharingan. But this is just a wild fanfiction idea.
In some fanfictions, it could be due to a special jutsu gone wrong. Maybe a jutsu that Kakashi was using to transfer some of his chakra to Naruto for a mission accidentally transferred his Sharingan power as well.
Naruto's perspective on the ninja world could change. With the Rinnegan, he would see things in a different light, both literally and metaphorically. He might have to deal with new enemies who are attracted to his new power or new responsibilities that come with such a powerful dojutsu. His relationships with his friends and allies could also be affected as they may view him differently now that he has this extremely powerful ability.
They could develop a mentor - mentee relationship. Itachi uses his knowledge of the Sharingan to teach Naruto new combat techniques. As they train together, they bond over their shared experiences of loss and their desire to protect the people they care about. Naruto might also help Itachi come to terms with his past actions and find some peace within himself.
In this kind of fanfiction, the author might come up with a convoluted plot. For example, Itachi could have discovered an ancient technique that allows him to bestow the Rinnegan on Naruto. It might involve sacrificing his own ocular powers in a way that defies the normal rules of the Naruto universe. This would be a very creative and out - of - the - box idea, but that's the beauty of fanfiction.
Well, in a fanfiction where Naruto is trained by Itachi in Sharingan, Naruto would first have to adapt to the new power. Itachi would probably start with the basics of how to activate and control the Sharingan. Since Naruto is used to his own physical and chakra - based training, this would be a challenge. However, with Itachi's guidance, he could master it. Once he does, he could use the Sharingan to analyze his opponents' fighting styles better. For example, he could quickly figure out the weaknesses in an enemy's jutsu and counter them effectively. He could also use the Sharingan's visual prowess to enhance his reaction time during battles.