The basic rule for keeping a Journal is that there are no rules! Whatever way you want to keep your Journal is a good way. You are unique and your Journal should be whatever you want it to be.
Be open to experimentation in your journal. Maybe ask God to show you how to use your Journal. If you are faithful to keeping it, it will probably take on a life of its own. My daily entries are usually a mix of what I have done or need to do, gratitude for whatever happened that day that I want to thank for, and sometime a relection on my prayer or something that I have read. I never write more than a page and often it is less than a page.
I do believe that keeping a Journal helps us to live a more reflective life. God has a plan for each of us. Our Journals help us to know what God wants. We need to be honest in our Journals. But we also need to enjoy keeping a Journal. Maybe my taking time off this January will help me to deepen my entries now that I am back to writing. I know that I do not need to write every single day, but it is a habit that I have had for so long that I want to return to daily journaling.
I try to get my students to Journal and they say it is helpful for them, too.