It depends on how you like to think, Naggy. For example, if you take a look at like most of my posts, they tend to be rather lengthy and can often times be very redundant in themselves. This is because I like to engage my audience/listener with constant stimulation of thought, making sure to drive home core concepts and ideas throughout my explanations. This is because I naturally need said stimulation to properly analyze and come to conclusions with my own way of thinking.
If you were to take a look at one of my notebooks, you'd see very disorganized, jumbled bull**** that makes no sense to any body but me. It's like that on a lot of pages, and a lot of pages in between are left blank. I don't bring my notes to tourneys, because I just don't have the room in the venue to comfortably take physical notes there, and in my own opinion, I feel like it can detract from my learning. Instead, what I decide to do, is I'll come home, throw on an album side, and just spew out as much information as I can into my notebook before the album side ends. Once the music stops, so does my pen. Over time, this has forced me to categorize my ideas into broad concepts, and then narrow my thoughts into the most important things I've learned that day, and then, if I have time, minor details that don't mean as much for me to be learning at my current level of skill.
After I comprise all of the thoughts for a couple of weeks, I take my notes, study them over, and try to remember what they all mean, and if I can't make out what my notes are exemplary of, I disregard that little blurb and consider it useless information at the moment. Everything else I take and reformat into paragraphs that are clear and concise and explain things in layman's terms so as to be as thorough as possible with them. Once I do that, I'll study the material for a while, and then I'll often times destroy the notes, because I'm weird and I've always had this mindset that maintaining notes in physical form will lead me to depend on them and I won't be able to consistently recall information when needed. You don't have to do that part, I did it with school work and stuff too, and it works alright for me. You don't have to do anything I've written above, because it might not suit your style of thinking.
Maybe you like to categorize things into neatly formed tables of info. Maybe you like doodling something next to a nice 2 sentence recap of an idea. Your notebook shouldn't look anything like my notebook, or anything like the next guy's notebook. It should look like your notebook, and should only matter if it makes sense to you. As far as how to start, it's pretty simple. Ask yourself one question. It can be any question at all (so long as you can relate it to smash). Which way is [player] fond of teching? Does [player] have a preference of side of the stage? How many exchanges did it take in the neutral before a stock was taken? Expound upon these questions, in whatever way you'd like. Try to explain things, try to think about how to beat things, and how to avoid being beaten by other things. Just think, and then write. It's a flow of consciousness, just write what you think.