For maths, by far the most efficient strategy I found for studying was doing practice questions. As many as reasonably possible. This not only builds up your ability to sort of act like a "human computer", completing a set of instructions consistently and quickly, but also exposes you to a ton of different ways that questions can be phrased, so you're not caught off guard by a weirdly communicated question.

It's also just a great way of overcoming the "yeah, I think I know it" trap. You see this kind of thing all the time - where people are like "Before the exam, I thought I had a pretty good grasp of the subject, but then I sat down to take the exam and looked at the questions and I just had no idea how to figure it out/write down a good response, as if all the things I studied left my brain in an instant. What the hell?!" That's a good indicator that you convinced yourself that you knew the material - you could sit down, read a key word like "osmosis" or whatever, and go "oh, I know that's a key word!" but didn't actually know the material, you were just recognizing it and reading the explanation for it and saying "yeah, I recognize this definition, I've read it before". There's a lot of ways to do the process of reproducing the material in your head, such as creating flashcards or posters (another comment here is about teaching it to other people, or at least imagining that you are) or indeed, practice questions, and all of them are better than passively reading and recognizing it. Just pick your favourite.

Other than that, make sure you realize the value of breaks - they are just as important as the time spent studying, as it gives you time for your mind to rest and organize what you've learned before pushing on. There's a lot of different ways of organize taking breaks which work for different people, but pomodoros are among the most popular (25 minutes of work, 5 minute break. Do that four times and you get a 30 minute break. Then repeat from the beginning). There's others, like one where you work for 50 minutes and take a break for 15/20 minutes, as well as one where you work for about two hours and then take a half-hour break. Whichever one works, works.

6 months is plenty of time if properly utilized; usually when people start panicking about exams, it's like "oh yeah, it's in 2 weeks" and that involves very different strategies. As long as you start now and use the time well, you should be totally fine.