I always test my journal reports in Preview Sandbox because there will be less noise in there. I start with a new journal entry report and my only filter is that the entry date is today. After this I will know that whatever comes up on my report is probably my work unless someone else is in the sandbox messing around.
After this, you can go into any project and start logging time. You'll notice everything you do will generate a new row on your report.
I don't use any text mode, because I can't really be bothered, so usually I throw all the fields available into the report and see which fields populate and with what data. Those are the ones I finally end up using, and through using those fields, I'm able to determine the type of filter I need.
For example, if the "field name" column of my test report populates with the word "hours" in it, I know I can add a filter for "field name" contains "hours" if I want to see more of those in production.
