A few things I've noticed about journaling as a writer.
The journal entry you write at 6 a.m. and the one you write at 11 p.m. come from two different people. Morning entries tend to plan. Night entries tend to confess. Most writers I know need both, but only do one.
Joan Didion wrote that we keep notebooks to remember "how it felt to me." Not what happened, how it felt. That distinction matters more than it looks like it should. I've gone back to old journal entries where I recorded entire days in detail and felt nothing, then found a half-sentence about the light in a stairwell that put me right back there.
Journaling for writers is weirdly hard to talk about because it sounds like advice your therapist gives you. But the function is different. You're not processing emotions, or not only that. You're training yourself to notice what you notice.
Virginia Woolf's diaries run to five published volumes. She wrote in them almost every day for decades. What surprises people is how much of it is petty, dull, contradictory. She complains about dinner parties. She's jealous of other writers. She changes her mind about her own books from one week to the next. The diaries aren't a record of genius. They're the compost that genius grew in.
Short entries work better than long ones. I don't know why this is true, but every time I commit to writing "at least a full page," I stop journaling within two weeks. When I let myself write three sentences and close the notebook, I keep going for months.
John Steinbeck kept a journal alongside East of Eden, writing in it every single working day, and a lot of those entries are just him warming up, complaining about his pencils, worrying he'd lost the thread. He published it later as Journal of a Novel. Reading it changed how I think about the relationship between the writing you show people and the writing that makes the showing possible.
There's a version of journaling advice that says you should write about your goals and visualize outcomes. I've tried it. It made me feel like I was filling out a form at the dentist's office.
The best journal entries I've ever written are the ones where I started with one thought and ended somewhere I didn't expect, where the writing itself was doing the thinking and I was just trying to keep up with it, which sounds mystical but really it's just what happens when you stay on the page long enough to get past the first obvious thing you were going to say.