Start with a topic audit
Print the specification or a topic checklist and rate every line red, amber or green. Be honest: green means you could answer an exam question on it today, without notes.
Revision time goes to red first, then amber. Re-doing topics you already enjoy feels productive and changes nothing.
Practise the way you will be tested
Maths is learned by doing. For each weak topic, watch or read one worked example, then complete a run of questions that get steadily harder, then try real exam questions.
In the final fortnight, sit at least one full paper under timed conditions with only the equipment you will have in the hall.
Mark it like an examiner
Use the mark scheme to see where method marks come from. Many students lose marks they had earned by not showing a line of working.
Keep a short error log: the question, what went wrong, and the corrected method. Re-attempt those questions a week later.
After the mock
The grade matters less than the question-by-question breakdown. That breakdown is the most accurate revision plan you will get before the summer.