I taught dd maths using the Letts revision guide and workbook. I used to go over about 3 sections of the book each lesson, and repeat each section three times, moving one section on each time. I used to have an A4 pad of graph paper to scribble down examples for her to work through as we went through each section. Each lesson something new would be introduced. It did not matter if she did not understand it, as this was just first time through. Children learn so much easier when the anxiety of not being able to understand is taken away. Also, IME, the best way to learn maths is to walk away from it and do something else, because the subconscious uses that time to work on it. Then when you get back to it, it seems easier.
She would then do the workbook by herself. There were some bits of the syllabus I struggled with, but happily Khan Academy has clear explanations on most topics.
Nearer to the exam, it is worth doing as many papers as possible and getting your dd to look through the mark scheme and examiner's report. She can then see where she is loosing marks and how she could get those extra few marks which could take her up a grade or even 2.
Lastly, trigonometry is something I remember struggling with at school. I hated just learning the formulae with no real insight into why they worked. Here again, Sal Khan, who set up Khan Academy, is very much pro an understanding of why and not just how being taught in maths. It could be well worth your dd looking at Khan Academy videos on trigonometry to se it suits her learning style. If she does, remind her to pause the video to try to work through the examples. People learn so much more by doing than by being told.