I think that keeping a record of the C depends on what you want to record and why. there are many sorts. You could for example, put together a scrapbook of photographs and all the bits of paper that we accumulate on the way, receipts, tickets, maps, etc. We did that and it is a great aid to memories and very useful in showing people who might want to do the C. It is also a great source of information. The only disadvantage is the fact that you have to carry all the bits of paper along, I generally just chuck them into the bottom of the rucksac and then when I get the photos printed back at home, sort them out by day. A scrapbook is a semi-public record, both in terms of what you record and what you show to people afterwards
A journal is more private however, recording not only what happened but maybe how you felt and what you did . A very useful daily framework for a journal is 'What Happened, So What, Now What' if you want your journal to be a record of your learning and maybe changes.
Another way of course is to simply photograph a lot, if you are a good photographer or maybe even record your impressions and the sounds, some people like sounds or images more than written words.
It may be that your decision as to what/how to record might depend on why you are going to the C. Is it a travelling adventure or a voyage of self discovery? Ask yourself what do you want to record and why and then maybe the how will become clearer. Personally I found I wrote much more on my first two week stint on the camino and by my fourth visit, I was writing very little though there were still things to work out.
Anyway do what ever feels best and change it if you want, there is no obligatory way to do this
Buen Camino