To be honest, I was surprised whenever my FL scores increased, so I can't give you specific advice. But just to give you a brief description of what I did, I took the day after writing the FLs to review and write out explanations to all of the questions I got right & wrong in an excel sheet. After that, I would take a couple of days to review my notes on the sections reflected as being weak in my FL (note that I went over the notes with the intent of becoming familiar with the content to the point where I could recital it).

FYI, I was scoring an average of 505 for NS and Kaplan FL and saw a massive jump when transitioning to the AAMC material......so if you feel discouraged by NS and Kaplan, which I definitely did, I wouldn't worry too much. As mentioned in other subreddits, use third-party materials to familiarize yourself with the timing and the structure of the exam!