Very different subjects indeed, I also took physics which I usually don't mention because I only got a C (without trying tbh), just to say that I to a degree can understand the challenges that science based subjects can pose.
For my other subjects, I was lucky because further maths was a natural extension to maths, so it was like only revising for one A level, at least that's how I found it. Fundamentally what it came down to was thoroughly understanding the scheme of assessment and what I needed to achieve in each exam to achieve an A* overall. Once I thoroughly knew what I needed to achieve I worked to ensure I was 100% confident with the material, this is where textbooks become very useful. After that it really just came to practicing past papers, and completing any and all related questions until I was 100% confident on the exams. Being 100% confident in an exam seriously reduced exam nerves. To be honest, that was pretty much it, you just have to invest the time into it, and it takes a lot of time, effort and energy but the results of your hardwork are worth it, or at least it was for me. I went from plans of attending a local below average university to study computing to attaining admission to one of the best universities in the world to study Mathematics with Economics.
I think the best part of this 'journey' is that I never really told anyone what I was doing, my teachers probably noticed an increase in my performance during class, homeworks, and assessments, but that was it. So my friends were just expecting an average set of results, so me turning around to say I got 3 A*s was an amazing feeling, especially seeing them look at me like "wtaf Jack?"
One extra little piece of advice for when your exams come round, I would always get into college 1.5-2 hours prior to my exam (even those pesky 9AMs) and lightly read through my textbooks and just chill out really. Whilst doing this, I saw a lot of people cramming questions in a blind panic and freaking out. I recommend that you do not do any practice questions on the mornings of your exams, at this point you don't stand to gain anything from it, the only thing that could happen is you finding a question you can't do which freaks you out for your exam.
I think that's pretty much all the advice I have to offer. Hope it helps, and Good Luck!