Hello, transgender person here!
Whenever people ask how to write a trans person, or an autistic person, or a woman, or a man, or a nonbinary person, or a 6th century scandinavian heathen, or a dentist, or indeed anything, I just say, first and foremost, you're writing a person. Now because you are a person, and you're surrounded by people every day of your life, you know how people are like.
And people come with details. Often details that are important to them. So when you are writing a trans person, you're writing a person, who is trans. By saying it is a detail, I do not mean to imply that it is trivial, or tangential to that person's identity, or that you should ignore it, because it could of course be central.
I don't really know how to word this, but basically, in all cases, you are writing a person. And you already have lots of experience in being a person, and in knowing other people, which means you already have a head start.
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also, my rebuttal to "what does having a transgender protagonist add to the story" is this: "what does having a cisgender protagonist add to the story?" there is always this underlying assumption that if your protagonist is trans, that must add something to the story, while if your protagonist is cis it doesn't have to add something to the story, which is a double standard, and is unfair.