One way could be through a kekkei genkai transfer jutsu. If a powerful ninja, perhaps an old hermit with knowledge of ancient jutsus, decides to transfer Sasuke's Sharingan to Naruto for the greater good of the village. This jutsu might be risky but in the world of fanfiction, anything is possible.
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.
In some fanfictions, Naruto gets the Sharingan as a gift from a dying Uchiha. This Uchiha, realizing Naruto's importance in uniting the ninja world, gives him the Sharingan. Naruto then uses it to understand the Uchiha's pain better and tries to bridge the gap between the Uchiha and the rest of the village. He also has to train hard to master the different levels of the Sharingan, like the Mangekyo Sharingan which comes with its own set of risks and rewards.
In fanfiction, Naruto could get the Sharingan through a blood - related connection that is revealed later. For example, it could be discovered that he has some distant Uchiha ancestry that gets activated somehow. Maybe a powerful enemy's attack awakens this dormant Uchiha blood in him, and as a result, he develops the Sharingan. It could also be that a powerful sage or a new character uses their own unique powers to bestow the Sharingan upon Naruto for a specific purpose.
In a Highschool DxD OC and Sharingan fanfiction, you can incorporate the Sharingan powers in the story by making it a source of mystery and envy among the other characters. The OC could use the Sharingan's ability to enter the minds of others (a more advanced form) to gather information about the devils' plans. This would make the OC a valuable asset to their group. Also, the Sharingan could have a connection to some ancient prophecy in the Highschool DxD world, which would add more depth to the story and make the powers more significant in the overall plot.
Naruto might adapt by first trying to understand the basic functions of the Sharingan. He could start with the simple ability to see chakra flow more clearly. Since he has always been perceptive about people's emotions, he could use the Sharingan to read the emotions of his enemies through their chakra, which would help him in battles.
Sakura's character would develop in terms of her combat skills. The Sharingan would allow her to analyze her opponents' moves more quickly and accurately. This would also boost her self - esteem as she would no longer be seen as just a support ninja. She would start to take on more dangerous missions and grow as a leader within her team. For example, she could use the Sharingan to see through genjutsu more easily during battles, which would give her an edge. Additionally, she might develop a new sense of pride in her abilities that was not there before, as she now has a unique power that few others possess.
Maybe by creating false visual illusions. For example, using some basic ninja techniques like the Transformation Jutsu to disguise as someone important to the Sharingan user. This could make the Sharingan user misjudge the situation and fall into the trap.
Well, in fanfiction, one way could be that some Kumo ninjas are given the Sharingan as a gift or a curse from an ancient being. They then start to form a special group within the village dedicated to understanding and mastering the Sharingan. This group would share their knowledge with others, and over time, the Sharingan becomes an important part of Kumo's ninja heritage, with its own set of training methods and traditions.