Imho, I would actually caution starting too early. That backfired for many people in my class. Usually people start from January to March, so you definitely don't want to burn yourself out by the end of December. If I could do it again I would personally buy Optoprep and the KMK basic course. I think Signature gives a false sense of confidence. The schedule was extremely hard to keep up with so not many people were able to follow it due to also being in class and clinic at the time time. Most people I know including myself completely skipped the flashcards and practice questions, which was a big portion of the course.
I'll also say the most important thing leading up to the last 2 weeks before the test is working on stress management and confidence. If you freak out during this test, it will be a bad time because the test takes a lot of mental energy. Make sure you practice lots of math problems as well so that you don't get bogged down on the exam. Force yourself to get up at 6am every single day the month before the exam, so that your sleep schedule is in check by the time you take it. Do light casual studying the week before, because if you stress study you will end up freaking yourself out.
Also, make sure you review many of your school notes and any possible books that you may have used over the past couple years. A lot of people said they wished they did this more.