I think writing proofs is more about structuring your argument in a logical way. Usually A-Level proofs are straightforward.
Let's say you want to prove that "An odd number times another odd number is odd".
Usually before I do anything, I start my first line with
"Let x,y be odd integers. We want to show that xy is also odd."
Yes, you're just re-writing the question again, but it's actually helpful (for me at least) to identity (i) what is the assumption - the "let" part; and (ii) what is our goal - the "we want to show" part.
Then from here, write down the definition of every keywords that needs to be defined. Here it's relatively straightforward, the word "odd" need to be defined. Of course you can go with the definition that "n is odd if 2 doesn't divide n", but you then need to say what "2 divides n" means.
Then it's a case of just try stuff. Now we don't really know at first glance a direct method is sufficient (this should be your first thing to try), or we need "more fancy" tricks like contradiction or induction. But the key is to just try. Usually you don't have much sensible options to go from the definition. If you get stuck, start from your goal (i.e. answer the question "what implies your 'we want to show' bit?"), and hope you can bridge between your assumption and your goal.
But the moral here is pretty simple: Know your definitions.