The only time I can think of when you would use a phrase like "by occupation" or "by trade" (or whatever) is when you want to draw a contrast between what you do to earn money, and what you do in your life that you really enjoy.
I'm an accountant by trade but I'm a painter at heart. I'm a writer by occupation, but I'm really a mom.
But otherwise, as Mr. M. says, if you just want to say what your job is, you just say it, period.