If you missed the other posts here are the links:
Google Earth Pro Tips & Tricks: Part 1 - The Basics
Google Earth Pro Tips & Tricks: Part 2 - Coastal Data
Google Earth Pro Tips & Tricks: Part 3 - Freshwater Data
Depending on your thought process and how you prefer to organize things there is some freedom in how you set this section up. If you mainly fish a handful of locations you can set up your folders by location. Since I fish many locations along the Texas coast I have my journal broken down by months.
To add folders right-click on the "My Places", then select "Add"-"Folder"; I titled mine "Fishing Reports". From there it's a lather, rinse, repeat to add (in my case) the month folders.
After you set up your initial group of folders you can begin to add individual reports. Again, there is freedom in how you would prefer to set this portion up, but I chose year then day then location, with the most recent report at the top.
The most basic way to add an entry is to right click on the month folder and add a "placemark" or use the "Add a Placemark" tab towards the top left. I use this when I go wade fishing and don't have a GPS track. Most of the time I do have a a GPS track, so I import that file after downloading it through Garmin Basecamp since I use a handheld Garmin GPS.
With the balloon open and within the "Description" portion is where you'll add any pertinent information you want to document for each trip. I keep information regarding meteorological, solunar, tides, currents, lure thrown, fish caught and a brief summary.
To keep everything organized you'll need to add some basic computer language, but nothing more than what's found on TKF. Highlighted in yellow is "<b>" and "</b>". This is how to bold text, as I've done for the headers. Highlighted in pink is "<br>" is how to move text to the next line. This is important as Google Earth won't automatically do this, so instead of neat, organized data you'll end up with one long paragraph. There are tabs to add links, images, but I don't find that necessary.
I hope this post makes sense, but if I made too big a jump between steps or you have a general question please ask for clarification.