The gem of a piece of writing advice has been floating around since stylus was first put to clay tablet: write what you know.
So, if you’re a psychologist, write about psychology. If you’re a filmmaker, write about filmmaking. If you’re a teacher, write about teaching.
The problem with writing what you know is the ease with which you can get caught up in writing about the minutia of your subject matter. I wrote a three-hundred-plus page novel about a teacher that was so detailed on the subject of teaching that it bored the pants off of anyone who made the mistake of agreeing to beta read it. It was terrible. I had to abandon the manuscript. Out of all those pages, I managed to salvage one chapter and publish it as a short story. Not a great showing for a year’s work.
Here’s the link to that story, if you’re interested.
A better piece of advice would be this: write what your audience would be interested in knowing. Gloss over the boring stuff that no one cares about. Talk to people about what you know. Guage what interests them and what doesn’t.
It’s okay to write about what you don’t know, too. If you want to write about something about which you know little or nothing, then do your homework. As you research, the interesting bits will stick with you, and you can gloss over the rest.
And that’s my stupid writing advice.