Harry can start by being completely honest with Draco. He should sit down with Draco and tell him about all his recent activities, who he has been with, and why. For example, if he has been working on a project with a classmate, he can explain the details of the project.
He could start by being completely honest with Ginny. Sit her down and tell her everything he has been doing, leaving no details out. For example, if he was working on a project with another character, he should explain the nature of the project, who else was involved, and why he had to keep it somewhat of a secret.
In the context of a fanfic, Magnus might use his magic in a positive way. He could create a magical bond or a truth - revealing spell between him and Alec (with Alec's consent, of course). This would allow Alec to see into Magnus' thoughts and feelings, and he would know for sure that Magnus is not cheating. Magnus could also start spending more time with Alec and less time on his own or with other people, at least until Alec's suspicions are completely gone.
It could be that there are some rumors going around. In the wizarding world, rumors spread like wildfire. If someone saw Harry in a compromising situation with another person, even if it was innocent, and told Draco, he might believe it without asking Harry for his side of the story. Also, if Harry has been secretive about some of his activities lately, it could fuel Draco's suspicions.
In some fanfictions, Harry could rely on the help of his friends. Hermione, with her vast knowledge of magic and Hogwarts' rules, might find some overlooked evidence. Ron, with his loyalty and sometimes unexpected insights, could also play a part. Together, they could present a solid case for Harry's innocence.
In many fanfics, misunderstandings like this can drive the plot. Maybe Draco saw Hermione spending a lot of time with another character and jumped to conclusions. It could be she was just working on a project or helping a friend.
If Harry has any special magical abilities or items that were overlooked, he could use them. For instance, if he still has the Marauder's Map, he might be able to find hidden passages in Azkaban to communicate with the outside or find evidence within the prison that clears his name. Also, his relationship with magical creatures like the Thestrals might come in handy. If they can be made to testify or help in some way, it could be a way to prove his innocence.
Well, it could be that Harry has started acting really distant from Ginny. He might be canceling their dates frequently or not being as affectionate as he used to be. In the fanfiction, Ginny might also find some strange gifts or notes that she suspects are from another girl to Harry, which makes her think he's cheating.
Perhaps Draco tries to win Hermione back even though he thinks she's cheating. He could do things like buying her gifts or doing favors for her, all while being angry and hurt inside. This would create a very complex emotional situation for both characters.
She might directly ask him about her suspicions. Just come out and say 'Harry, I think you're cheating on me. I've noticed you acting strange lately.'
Harry might be confused at first. He doesn't understand why Draco would think he's a doll fan. So he might just shrug it off and not pay much attention, thinking Draco is just being his usual difficult self.