Keep a log for 20 ton or less there is no size limits, but it is time at sea, so anchor, or dock time doesn't count, only time underway. Things change, but when I got my license they accepted a written log of time and dates copied from my personal logs to an official seatime form and signed by boat owner, (could be yourself). Some of my time was a 20ton yacht, some was a small runabout, some was a dinghy. As long as I could document 4 hours or more underway on each of these vessels iat a time, it was counted. You can call their licensing division with any questions. I had several regarding counting fishing time, and what Game warden/USCG encounters I was required to document. They happily answered all questions and worked them out with the officer in charge of my case.
Get everything in writing, if you go out on the water write it down. Take a license class when you are ready, they have online tests that you can use to practice. Put a package together and try to submit it. If they reject you, they will well document the reasons why, then you will be better prepared for the next time. Good luck!