How do you avoid making silly mistakes in a maths exam?

In my experience practice makes perfect and the best way to do this is to immerse yourself with as much past papers as you can do. Maths is maths so don't be afraid to check other exam boards for their past papers as long as it's mostly similar. This not only reinforces revision but it you'll nail down all the important keywords and procedures needed in the process when checking the answers. Just like how actors and olympians practice, you'll become more comfortable with the material and confident and enough not to make any mistakes because you've rehearsed even the hardest style question perfectly. This means you're less likely to stress and be more efficient during the exam which means more time to meticulously check your work for mistakes.