Effectively studying for maths involves gaining a good understanding of the concepts/content and applying this understanding to practice material, such as textbook questions and past exam papers. Doing so will ensure that you are able to apply your knowledge to various types of questions while simultaneously determining areas where improvement may be required (if applicable). In such a case, you should ensure that you regularly seek further clarification regarding particular concepts that you feel you may not be fully confident/familiar with. Once you consolidate your understanding of those, you can continue your studies and preparations by completing more practice material.
Regarding textbooks, there seems to be a general consensus that Cambridge is the best textbook that you can use due to the fact that it includes more difficult questions compared to other textbooks such as Maths in Focus. Based on this, you may wish to rely primarily on the Cambridge textbook.
The use of Maths in Focus appears more appropriate when acquainting yourself with different mathematical concepts, whether it is through consulting the explanations of particular concepts, or when attempting questions relevant to those concepts, allowing you to become familiar with the content. Because of this, another method would involve using both Cambridge and Maths in Focus, as follows:
Using Maths in Focus as you first encounter a particular concept can assist you in familiarising yourself with that concept.
Subsequently, using Cambridge will enable you to master that concept, thanks to the exposure to more challenging material, which constitutes good practice.