Hey @hellograduate ! I wrote some suggestions on a similar post, so I’ll just copy and paste:
Critical Reading used to be my lowest as well, but I took a test preparation course called Testmasters, and I would recommend you looking for something similar as well. One tip I learned for reading is that the passage questions, besides the overall theme/purpose ones, are IN ORDER. Meaning, although there are some questions with line references, the ones that don’t are in between those that do. This also means that the ANSWER is ALWAYS in the vicinity of the paragraph(s) you are focusing on in relation to the question ordering. This leads to another tip - never assume. The SAT likes to trick you into thinking some answers are correct, even if the answer is not directly from the passage. Remember, you can always find the answer in the passage!
My Writing used to be my second lowest, but I raised to be on par with my Math, my best subject. For writing, there are error patterns that can easily be spotted. Some common ones are parallelism, pronoun/verb agreement, idioms, etc… You can look for practice online anywhere for writing. Once you get hold of the writing, it becomes so much easier. As for the essay, don’t do a structure, like intro body (3 supports) and conclusion. I did that one time and I got a 9. Try writing 2 supports and 1 rebuttal instead of 3 supports - that’s how I got my 11. And as for the examples - just make them up. You don’t have time to think about what to write - whatever comes up on your head, you write down. I once wrote about a Korean drama I watched that I turned into a “historical movie” and Avatar the Last Airbender. Worked great. It also helps to know some common classical literature/historical figures as well, like Huck Finn, Scarlet Letter, Abraham Lincoln, MLK Jr., etc… Write slightly larger than your normal handwriting so it nearly fills the two pages it provides - this may sound unnecessary but I heard it gives you a bump of 1-2 points.
Math is honestly the easiest to improve. I can’t tell you how to specifically improve besides to practice. Or maybe one thing - make sure you don’t make any mistakes. And another - if you’re stumped on a problem, this is what I thought to myself: all problems can technically be solved without a calculator, so if you can’t solve it, then you’re overthinking it. Also, try solving the problems super fast so you can re-solve them a second time - this helps catch any silly mistakes.
Practice, practice, practice. It helps you familiarize with the test, so you automatically know what to expect.
Note that these suggestions are for the old SAT - I know nothing about the new SAT 
Hope I helped!
