Taking notes about karate training is very important.
My Sensei suggested to me a very long time ago that I should always keep a notebook with me when I go to the dojo or karate training events so I can write down the important thoughts. I have tried to follow this advice ever since. I usually have one with me at all times and I try to capture as much as I can about what I learn.
I find years later that these notes are invaluable. They provide me insight into the stage of development I was experiencing at the time. They also help me remember details about a technique or principle I may have forgotten or since then. Sometimes, they even help me to make a leap in understanding or a small revelation.
I usually don't write long detailed entries about a subject, but rather organize my thoughts by bullet items or short statements where I try to get to the heart of the matter. Then, I try to supplement these with sketches and stick figures.
I always write in the first person and try to note other things that were happening at the time in the world. This helps me remember the time-period better than a date, although I still date each entry.
Sometimes I don't finish an entry because I got busy with something else in my life. Then, a few years later as I'm reading my entries, I will try to complete the entry as best I can from memory. I read my entries often and write notes about how my understanding of the subject has changed. I think this is very helpful.
I have pages in my karate journal that I have left intentionally blank to put pictures relevant to the event. I have the pictures, but I just haven't put them in yet.
Don't get me wrong, I may sound like I'm very disciplined at journaling, but I'm not. I have far too many journals, notebooks and sketchbooks that I have started to write in but then lost only to be found again years later in a box or on a shelf. As I said, I don't always finish an entry or a thought.
The important thing is that I always have one with me when I'm training in case I need to write down a thought.