I find that physically writing down something with a pencil is the best way, for me, to learn. I find it reinforces your learning, and from a psychological point of view; it helps to reinforce the connections the brain constructs when learning something new. The main goal or aim of a journal is to keep you focused. 

The second most important point to remember, when writing a journal, is the revision value it contains within its pages; the facts, analysis, and diagrams you studied.