You can write whatever you want. A single word/feeling, a rant, an insight that came to you while staring at the wall. You can write about your day, what you want to do, how loud the neighbors' lawn equipment is. The important part is that it's taking thoughts from your brain and putting them down on paper.

Journaling is just like any other technique - it works best if you make it a habit. Same with meditation, breathing, mindfulness, etc. Do it when you're feeling okay. Any time you think of it. You can use a physical reminder if (like me) you have trouble remembering to Do Thing(s). Like take a can or a box and put it upside down in a conspicuous spot. When you see it, jot something down/type it out, then move the object to where you'll see it later. But the goal is to do it enough so that when you are feeling overwhelmed, it's something that you can do with minimal cognitive load. You need to create that pathway first before your neurons will go there on their own.

Practice makes ~~perfect~~ better.