Every day you learn something new. It might not be mind-blowing, it may not "change your life" as every self-improvement book seems to tout, but it is new information.
Quick: what did you learn 6 days ago? Don't remember? You're not alone. Let's address that, shall we?
The TIL ("Today I Learned") Journal is a simple concept. Every day, you write down one thing you learned.
That's it. You have everything you need to start.
How you write out and maintain your TIL Journal is up to you. You could use a physical notebook or something digital like Obsidian, Evernote, or even a text file in Notepad (a coworker of mine has a yearly text file he uses for this exact purpose; nothing fancy).
Personally, I have been doing something similar with my personal wiki, Everything I Know. It's a dumping ground of interesting things I learn about throughout my life. Some days I'll contribute five or ten things to it. Other days, nothing (although this is usually due to laziness and not actually learning anything).
My personal wiki was heavily inspired by someone else's digital brain, which I stumbled across about a year ago and was blown away by how much this person had contributed to their own little garden of knowledge. My immediate thought was, "This is so cool! I could do that". (Another example of why you should learn in public)
And so I gave it a shot. After a few false starts, it's now the one thing I consistently have open on every computer I work on. I refer back to notes in it daily.
I think the biggest benefit I get is a reminder of how much I've learned over time. Some days you may feel like nothing has changed in years, that you haven't gotten any better at your craft, or your stagnating. Your TIL Journal can help prove that voice in your head wrong.