I think the best way to start is to look at the official syllabus. It shows clearly what topics can come up, including the logic parts in Paper 2. Make sure you are comfortable with those, because they don’t always come up in A-level lessons.
For maths basics, keep your algebra, probability, sequences and geometry very solid. The questions are not about heavy calculations but about spotting patterns, so if the basics are weak it’s easy to get stuck.
Past papers are really helpful, but don’t just rush through them. After each paper, go back and think: what was the question really testing, and why did I miss it? If you collect those ideas, you’ll see that many TMUA problems follow similar styles.
One tricky thing is the format. No calculator and no paper means you need to train yourself to think steps in your head and only write the key parts on the board. At first it feels strange, but if you practise a few times it becomes natural. Doing timed practice papers is also a good way to get used to the pressure.
Overall, TMUA is less about long calculations and more about quick, clear thinking. The more you practise and review, the easier it will feel.