Like a hardcopy journal? If that was the plan, I would recommend doing it electronically. If you are taking notes on everything, you are going to fill multiple journals and if you are looking for a relevant sentence in all your notes, it'll be like looking for a needle in a haystack.
I've started keeping a notes file for every project, as well as a general one. That way, I can use command F to find it. Also, depending on your referencing software, it makes for a quick referencing job. I have found it has really helped (2nd year post doc for reference).
To your above comment on scientific writing, I highly recommending taking this course: https://www.coursera.org/learn/sciwrite. Even the free version is great.