Some people set aside a time each day to write in theirs. Others will write on and off throughout the day. Overall, the main thing to keep in mind is consistency. Thinking later that you should have wrote something down and then not being able to recall exactly what it was that you wanted written down will do you no good. Try to get in the habit of writing at least a few words or sentences a day in it.
Writers lead busy lives and actually forget about their journals.
You need to set a schedule in your mind to the fact that you have a journal and to use it. When, each day, you do this will be a personal decision. Where to Write Keeping two or three separate notebooks is perfectly acceptable. This will give you the freedom to write wherever it is convenient too. One by the bed for dreams and ideas that come to you as you are falling asleep or when you first awaken and have thoughts spilling out.