I have only taken the SAT once, and the score which I received was not at all what I was aiming for, primarily on the math portion of the test. My SAT is coming up and this may be the last time I can take it before the scores have to be sent to colleges- this may sound a bit broad, but how can I improve on the math portion and what should I know in order to do well? There are tons of review books and the collegeboard practice tests are helpful, but is there anything specific I should be focusing on in order to ensure a better score?
Is this for the Dec SAT? Just a note, studying roughly a day before isn’t very wise, especially with this being your last time. But anyway, take a few practice tests just in math to try to help raise it using the College Board’s Official SAT prep app (it scans their official practice tests found on their website and tells you what you got wrong and how to get the right answer). Take maybe 3 if you can between now and Saturday, but I would strongly advise you not to study the day before the exam.
Part of taking these standardized tests has to do with your mental health, too. Did you get enough sleep? Did you exercise enough? Cramming the day before can cause stress and might influence your performance on test day. So try to get in 2 practice tests tonight, and go over every answer you got wrong. Take out a sheet of loose leaf paper, write down in bullet points the CONCEPT you got wrong, and any formulas you forgot that you can use again later (like a cheat sheet you can glance over right before you walk in to take the test). Then, approach the questions you got wrong again with this new knowledge, and ingrain in your mind the right steps you can take to get the answer.
Take them in timed conditions, too, and look up online tips for mental math for the no calculator portion. SAT Math can be a bit tricky (for some reason the Nov SAT Math was way easier than I had expected, though!). When reading the questions, circle what you need to find, underline, mark up the whole test so you don’t do any unnecessary calculations or take any unnecessary steps that eat up time.
Remember: you can do this! I don’t think you’ll have a large increase in your score with so little time, but if you maintain a positive attitude and go over what you get wrong tonight, I know you’ll improve, and any improvement is a good one 
Good luck!
For the math portion, the thing that helped me the most was doing a lot of practice problems-- I mean A LOT! The SAT has so many nuances that can’t be adequately described in words, but rather they are best learned through experience that one can only attain through a multitude of practice problems. After I finished the 8 practice tests, I felt that I needed more practice, and as such I went out and looked for some other source to practice with. I found UWorld on Reddit with a free trial code and I found like 500 SAT Math problems that were almost the same as the 8 practice tests. So, I would recommend that you look through Reddit to find the UWorld free trial code (if you can’t find it just message me and I will find it for you). UWorld and the 8 practice tests are all you need to achieve a 750+ on the SAT. After a 750-770, it is really about luck and the test that you are given honestly.
Source: 790 SAT Math. 760 PSAT Math