I ended up building a product (https://www.friday.app/) to make this easy and automated. While it's built for teams to share updates and reflect (think weekly updates, retros, etc), there's "single-player" mode available too.
I like the digital journal format because I could never start the habit with paper. The automated reminders were critical to establish the habit. I still keep a regular notebook where I'll document thoughts, but it's more ephemeral.