How do I score band 9?

To answer this question, we should examine how an examiner is valuing your answer script.
The first criterion for valuation is task achievement. The examiner will be looking for your ability to answer the question accurately. This means that you should read the questions very carefully and make sure you cover everything it asks. You should answer all parts of the question, present relevant ideas with convincing explanations and support these ideas with authentic, specific examples.
The next one is coherence and cohesion. You can increase your band score by making it very clear to the examiner what each paragraph is about and then logically organizing each sentence within that paragraph. The key points in coherence and cohesion are structuring your answers in logical paragraphs with introduction, body and conclusion. One main idea per paragraph is preferred, and it should include an introduction and conclusion. Main ideas should be supported with an explanation and then an example. In addition, use cohesive devices accurately and appropriately, with varying linking phrases, using synonyms.
Lexical Resource is another area. You should try to vary your vocabulary using accurate synonyms, using less common, question specific words that accurately convey meaning, and check your work for spelling and word formation mistakes.
The final criterion is the grammatical range. Learn to use a variety of complex and simple sentences with appropriate structures. And, check your writing for grammatical errors.