He might use the invisibility cloak. Harry could sneak into Azkaban unnoticed and then use magic to break Draco out of his cell. The cloak has served him well in the past, and it would be a simple yet effective way to get past the guards.
In some fanfictions, Harry might use powerful magic he's learned secretly. For example, he could discover a new spell that can break the Azkaban's defenses. He then sneaks into Azkaban, locates Sirius, and uses this spell to free him from his cell. Maybe he also has to fight off some Dementors on the way out.
One possible plot could be that Harry discovers some new evidence that proves Draco's innocence. Maybe there was a frame - up by some dark forces within the wizarding world. Harry, being the hero he is, feels compelled to act and uses his influence and skills to break Draco out of Azkaban. This could lead to them having to go on the run together, hiding from the authorities while they try to expose the real culprits.
One major challenge is the Dementors. They are extremely dangerous and can suck out a person's soul. Harry has to find a way to either avoid them or defeat them. Another challenge could be the security measures in Azkaban. There might be magical locks, guards, and other barriers that he needs to overcome.
The plot could start with rumors reaching Harry that Draco is being unjustly treated in Azkaban. Harry, being the hero he is, decides to investigate. He uncovers a conspiracy within the Ministry of Magic that is trying to keep Draco locked up for political reasons. Harry then uses his connections, like his friendship with Neville who now has some influence in the Ministry, to get a hearing for Draco. During the hearing, Harry presents the evidence he has found, which includes magical artifacts that prove Draco's innocence, and Draco is finally released.
Well, in some of these fics, Sirius might time - travel to save Harry from a future threat. For example, he could see a vision of Harry being killed by Voldemort in a new and terrifying way, and then he uses time - turning magic to go back and change the course of events.
Another theme is the power of love. Harry's love for Sirius gives him the strength and courage to face the many obstacles in his way. It's a theme that's often explored in fanfiction as it was also a big part of the original books. In these stories, Harry's love overcomes things like the dark magic surrounding Azkaban or the threats from Voldemort's followers.
Yes, it's possible. Sirius is a popular character in the Harry Potter series, and many fans like to imagine him in different scenarios in fanfiction. He has a complex personality and a lot of untold stories, so fanfiction about him in the context of 'Prisoner of Azkaban' can be very interesting.
It varies in different fanfictions. Some have Harry at 12, while others might depict him as 14 when this event occurs. It depends on the author's imagination and the story's plot.