You write a journal to influence who you are becoming. You write down what you have done, or intend to do, or have read or have learnt today. Make plans in your journal.
Give emphasis to your own primary experience. Recording what you alone know is something nobody else can do. Your primary experience has no structure. You impose order on it by choosing what is important. If you can measure, count, compare or make categories, that's helpful. Your own self collected data is real knowledge and real strength.
When you really absorb your primary experience, reflect on it, understand it, you are better able to know what's real, true, or valid.
We don't live in an information society. We are fed with other people's propaganda and encouraged to believe it. As a result we live in a society that agrees to ignore the information, a society that constructs a fabric of mis-information or dys-information and agrees to call it "truth", or common sense. Your journal filed with your own primary knowledge can help you stand for reality against the tide of "me too" claims, or unthinking and self chosen not knowing. Such personal and grounded knowledge allows you to be strong when faced with fierce opposition.