If Naruto creates a lightsaber, it could be related to his connection with Kurama. Kurama's chakra is extremely powerful. It's possible that this chakra could be harnessed in a way that can mimic the energy source of a lightsaber. Naruto, being the creative and adaptable ninja he is, would then figure out how to shape and control this new form of energy into a blade, much like a lightsaber. And he would surely use it to protect his friends and the Hidden Leaf Village.
The plot could start with Harry finding an ancient Jedi artifact during one of his adventures. This leads him to become curious about lightsabers. He then uses his magic knowledge to try and replicate it. Along the way, he might face opposition from some in the wizarding world who think it's too dangerous or against wizarding traditions. But Harry, being the hero he is, persists. Eventually, he creates a lightsaber and uses it to defeat a new, powerful dark wizard who has emerged, using both his wizarding skills and his new lightsaber skills.
Well, in this fanfiction, Harry would probably use his magic to create a unique lightsaber. Maybe he'd combine his wand's magic with the technology of the lightsaber. For example, the blade could be made of enchanted light that can cut through dark magic as well as physical objects. He could use it against Voldemort's Death Eaters in a whole new kind of battle, with the swishing sounds of the lightsaber adding a new level of excitement to the wizarding duels.
Perhaps it's given to him by a mysterious figure. This figure could be from another dimension or a very old ninja who has been in hiding for a long time. They recognize Naruto's potential and power, and decide that he is the one worthy of wielding the black lightsaber. This could be part of a bigger plan to help Naruto defeat a new and powerful enemy that neither the ninja world nor the normal means of combat can handle.
Naruto's fighting style would likely incorporate more acrobatic flips and spins to take advantage of the lightsaber's reach. He'd also use his shadow clone jutsu in new ways, like having clones surround an enemy while he attacks from above with the lightsaber.
Another possibility is that the black lightsaber has some sort of special power or property that ties into the Naruto universe's existing powers. For instance, it could be attuned to chakra in some way. Maybe it can be powered by chakra instead of the typical energy source in the Star Wars universe. This would create an interesting dynamic as Naruto characters would have to learn how to manage this new form of power while still using their own ninja abilities.
Sasuke could be one to help. Given their history and Sasuke's intelligence, he might contribute some knowledge about energy manipulation which could be useful for the lightsaber's power source.
He could find it during one of his missions. Maybe he's exploring an ancient ruin and stumbles upon the lightsaber. It could be in a chest or just lying there, and Naruto, being curious, picks it up.
Well, in such a fanfiction, Naruto might use his chakra in a unique way to power the lightsaber. Maybe he'd combine his ninja skills with the new weapon. For example, he could use his speed to make quick slashes with the lightsaber, like his fast kunai attacks.
In the fanfiction, the double lightsaber might be a gift from an unknown entity. Naruto, being the curious and adventurous type, would start training with it. He'd have to learn how to handle the two blades simultaneously, which could be a whole new challenge. This could lead to new forms of combat that blend Naruto's existing fighting styles with the lightsaber techniques.