Regarding maths, it is important to be interacting with relevant material regularly in order to support your knowledge of the content. This primarily includes the textbook(s) that you use, as well as any other material (such as worksheets and worked examples targeting particular concepts) that you may have been given at school. If you find difficulty in understanding or improving your knowledge of specific aspects of the content using the material that you have, you may wish to consider watching videos that explain individual concepts, a lot of which are available on YouTube. As well as this, you can always ask content-related questions on BoS (this applies to both subjects).
Additionally, I will note that completing practice material (such as textbook questions as well as past exam papers) can also contribute to your understanding of the content. Regularly attempting relevant textbook questions in addition to past exam papers gives you the opportunity to apply your knowledge of the content to a variety of questions, allowing you to assess your performance potential, which is essentially underpinned by your knowledge of the content. Doing so will allow you to determine your strong points and will assist you in identifying potential areas of improvement, which may require you to revisit the content and ensure that you enhance your understanding.
I hope this helps!