Well, he might start by using his charisma. Naruto has always been good at making friends. He could reach out to old allies from his village, like Shikamaru and Choji. Also, he might convince some of the outcast ninja who have been mistreated in other villages to join him. His reputation as a hero who saved the world would surely attract a lot of people.
One challenge is trust. Not everyone will immediately believe in Naruto's intentions. Some may think he is trying to gain power for himself. Another challenge could be resources. Feeding, housing, and training an army requires a lot of resources. He needs to find ways to get enough funds and supplies.
Maybe it's a way to protect the village. The ravens could act as an early warning system. They can fly over Konoha and the surrounding areas, and if they detect any incoming threats, they can quickly report back to Naruto. It's a form of aerial defense that can complement the existing ninja guards.
Obviously, his friends Ron and Hermione would be among the first to join. Neville could also be a key member, as he has shown great courage. Then there might be the members of Dumbledore's Army who are still at Hogwarts. The Weasley twins could also be part of it, using their unique magical inventions to give the army an edge in battle.
He might use his charisma. Naruto is known for being able to inspire people. He could convince soldiers who were once loyal to Darth Raven to follow him instead. He may also introduce new training methods or strategies that make the army more effective, thus attracting more recruits.
In this fanfiction, Harry might start by gathering his most loyal friends like Ron and Hermione. They could then recruit other students who feel oppressed by the Death Eaters' regime at Hogwarts. Harry could use his knowledge of magic, especially defensive spells, to train the army. Maybe they'd also include some magical creatures, like the centaurs or the house - elves who are willing to fight for a better future at Hogwarts.
He could start by using his chakra to communicate with them. Naruto has a large amount of chakra, and he might be able to establish a mental link with the ravens. Through this link, he can give them commands and instructions.
One possible plot twist could be that Naruto discovers a hidden power within Darth Raven's army that he can use to his advantage. Maybe it's a special type of chakra or a unique weapon. Another could be that members of the army start to rebel against Naruto, believing he is leading them astray. This would create internal conflict.
Tsunade could play a crucial role. She could use her vast knowledge of resources and economics to secure the funds and materials needed for the fleet. Jiraiya, if he were still alive, would bring in his worldly knowledge and connections from his travels. He could help Naruto find rare and valuable materials from far - flung places for building a truly remarkable fleet.
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.