Putting down “writer” as your occupation on forms. It’s practically legal.
Wearing a tweed jacket with leather elbow patches. Or does this make you a college professor? I can never remember.
Smoking a pipe (Harlan Ellison only)
Paying someone else to write stuff under your name. This is actually legal.
Building an amazing writing robot you can dictate your stories to
Buying a nice pen set, especially if they’re fountain pens. And using them too, I suppose. For writing, that is, not, say, stabbing people. Unless you later write about it.
Lists count as writing. Yes, they do.
[Write something here. Ta-da, you’re a writer! This is different than Step 1 somehow.]
Changing your legal name to A. Writer. This, as you may have surmised, is legal.