Well, perhaps he uses some powerful magic he's learned secretly. Maybe he has help from unexpected allies, like a magical creature that can bypass the prison's defenses.
In the fanfiction, it could be that there are flaws in Azkaban's security system that Harry discovers. He might use his knowledge of the prison from Sirius's escape and find a new way out. For example, he could manipulate the Dementors in some way. Since Dementors are affected by strong positive emotions, Harry could use his love for his friends and family to drive them away and make his escape.
Maybe he uses some powerful magic spells that he has learned secretly. For example, the Patronus charm could be used to fend off the Dementors which are the main guards in Azkaban. He might also get some unexpected help from his friends or some mysterious characters within the fanfiction world.
It could be Sirius Black. Since he has his own experience with Azkaban, he might know some secrets about the place. He could return in some form within the fanfiction and assist Harry in his escape, maybe by leading him through some hidden tunnels or using some special skills he developed during his time in prison.
One reason could be that Voldemort wants to use Harry for his own gain. Maybe he thinks he can turn Harry to the dark side and use his power for some grand evil plan. He might believe that with Harry on his side, he can finally defeat all his enemies and rule the wizarding world completely.
One interesting plot point could be how Harry devises the plan to break Sirius out. Maybe he uses his invisibility cloak and some clever spells. Another could be the reactions of the other characters when they find out, like Hermione's concerns about the legality and consequences.
Perhaps Voldemort has some inside help. He could have bribed or threatened one of the guards or even a Ministry official who has access to Azkaban. This person could then help him get in and break Harry out. Or he might have found a secret passage into Azkaban that the Ministry doesn't even know about, and he uses that to his advantage.
Obviously, Harry and Sirius are the main characters. Harry is the one trying to break Sirius out, and Sirius is the one being broken out.
In the fanfiction, it could be that Daphne has some unique magical ability that she uses to protect Harry in Azkaban. They might encounter other inmates who are actually innocent too and together they start a mini - revolution against the unjust system of Azkaban. Harry, with his experience of facing Voldemort, might lead the way in finding a way out of the prison.
One possible plot could be that Harry and Ginny are wrongly accused and sent to Azkaban. They then have to find a way to prove their innocence while dealing with the dark and dreary environment of the prison. They might discover a conspiracy within Azkaban that is related to the real culprits they are being framed for.
In the 'harry locked in azkaban fanfiction', it's possible that Voldemort has a hand in getting Harry locked up in Azkaban. Harry, being the strong - willed wizard he is, tries to communicate with the other inmates to find out if there are any clues related to Voldemort's new plot. He also has to deal with the harsh conditions of Azkaban, like the cold and the constant threat of the Dementors sucking out his soul. But he doesn't lose hope and keeps looking for a way to escape and save the wizarding world from whatever danger Voldemort is planning.
One possible plot could be that Fem Harry grows up in the harsh environment of Azkaban, developing a tough exterior. She might discover unique magical abilities due to her unusual upbringing. For example, she could have an uncanny ability to sense the emotions of dementors, which she uses to her advantage. This could lead to her being a target for both the prison guards who fear her power and those outside who want to use her for their own gain.