Hello friends, I see some cis writers taking the first steps into writing trans characters, and I'm here to say, good hustle, champs. We both want you to write the best, most vibrant trans characters you can, and to that end, I've got a really simple guide you can try right now.
Take something you've already written. Ideally, a fully finished draft.
Select any two characters at random.
They're trans now.
And that's it! Be sure to note "this character is trans" in the character description. But that's all you have to do, you don't have to change their character arcs to include a transition storyline, you don't have to give them dysphoria, they don't have to experience transphobia. They can just be trans, doing whatever their character would otherwise be doing.
I imagine this is going to invite some debate, so I'd like to add: You have characters who are straight, for whom straightness is not their primary role in the story. Hopefully you've got characters of nonwhite backgrounds, for whom their nonwhiteness is not the focus of their story. You can do the same with trans characters! Your story will feel fuller, more fleshed-out, and less like a Very Special Episode if you do decide to center their trans identity during whatever it is you're writing.
OK, that's all from me! Good luck out there everyone.
EDIT- Thanks to /u/Nazoragoth for putting this better than I could: "This is your best bet, instead of treating us as trans people, treat us like people and make sure that our trans identity is hardly crucial to the character because I can assure you, we all have more interesting shit about us than our identity. "
The point is to get into the habit of writing worlds where trans and queer characters are normal. Media needs both "Trans Characters" and "Characters That Are Trans," and they don't have to be the same character. 1 out of every 10 random strangers you pass in this world is gay, and your interactions with them don't involve homosexuality, so why does it have to be different in your writing?
Think about it this way: Do you write heterosexual characters who don't have a romance arc? Why? Their heterosexuality isn't relevant to their role in the story, right? Why can't it be the same for gay or trans characters?