Writing transgender characters is the same as writing normal characters just with a few extra thinking points.
1. How far along in the transition is this person?
You might think that having your character come out to their parents, get kicked out of the house and then pass enough to be taken in by someone who they are then struggling to come out to is a great plot line but it’s not. Most people won’t pass very well before they are on hormones and that can take months or even years to start depending on where in the world your character is. So in short, know how long the transition prosses is and make sure your character doesn’t show traits (like a deep voice) to early.
2. How masculine or feminine are they?
Just because your character is trans, it doesn’t disregard that they were born as their birth gender. You can’t undo 18 years of social conditing so easily… its ok to be a girly guy or a boyish girl. But its also ok to be a really macho man too. Just consider how your character came to act like this and at what age they started showing these traits.
3. How open is your character?
Will they tell a stranger that they’re trans? Wear a pride flag? Or do they lie about every part of their past to cover the truth? Some people avoid public bathrooms like the plague and others are comfortable enough with themselves to go into either depending on the situation.
4. Who Knows?
Some characters are still coming out to themselves, some to their parents, some to their friends. But some people fall through the cracks. It can be a great plot device to have someone your MC hasn’t come out to yet crash the party. On the other hand, think of how each of these social groups found out. Did the mom tell the dad? Did most of the MC’s friends leave and now they have to make new friends?
If you have any other advice or would like a sequel that explains more directly ‘how to’ please let me know!