First of all, if your math foundation is not very good, you can use Tang Jiafeng's textbook. You can consider "Mathematics Review for Postgraduate Entrance Examination" as an available book. Anyway, you can buy Mr. Tang's books and learn from his teaching videos.
If you don't know what your math level is, for example, can you get 80 points (full score 100) in the final exam of advanced mathematics? If not, it means that your math level is not very good. Then if you can get this score, you can use teacher Zhang Yu's textbooks in the initial stage, such as advanced mathematics 18, linear algebra 9, probability and mathematical statistics 9.
It was done after you read the textbook above and finished the exercises in the textbook. Here, I recommend the title source 1000 by Zhang Yu, which is more difficult than the real title, but the difference is not big. At first, the speed of doing the 1000 question will be very slow, but it is normal, as uncomfortable as squeezing toothpaste, but after experiencing it, I feel nothing.
If you still feel that you are not very solid in mathematics at this stage, I recommend Tang Jiafeng's question 1800, because his answer will be more detailed. It is normal that you can't understand the answer to teacher Zhang Yu's question 1000. Don't be discouraged!
In the final stage, the most important thing is to do the real questions. Just focus on what you have been doing for nearly 20 years, and do it twice until practice makes perfect.