Can you provide details on how to obtain bands 7 to 8 in the IELTS Writing?

I will give you some guidelines for task 2 - essay writing.
1) Spend close to 10 minutes as your preparation time before you start to write.
2) Understand the essay topic and rewrite the same in two or three different ways using your own vocabulary. (once you understand the topic, you gain the confidence to explicate the topic ideas further) Use these sentences in your introduction, main body paragraphs and conclusion.
3) It is always better to start with a topic sentence, and other ideas/ sentences should revolve around this main idea. Provide the examples that you have gone through in your real life.
4) Use appropriate conjunctions to link the ideas/sentences and paragraphs. They should flow.
5) Read through and make necessary corrections such as spelling and punctuation.

To obtain band 7-8, you need to keep a few things in mind:
1- STRUCTURE- in all the writing tasks, be it an essay, letter, diagrams, bar graphs, etc. you have to keep in mind the structure. Each of these has a unique way to be handled. For example, in a formal letter, you cannot use an informal tone, and for a formal letter, you must not use an informal tone. Paraphrasing of the essay in the introduction will fetch you ya higher band. Also, the number of paragraphs in the essay should not be less than or more than 5. While writing the conclusion, don't exceed the word limit of 30-35 words. Be precise while concluding on which side of the argument are you. 

2- PUNCTUATION- since this is a test to measure your language skills punctuation play a crucial role to fetch you a higher band.

3- SPELLING- You cannot undermine the importance of learning the correct spellings. 

4-COHESION- Your points should be well supported and naturally move from one point to other. Also, avoid irrelevant details else you will lose marks. Avoid repetition to complete the word limit. Read and attempt as many essays, letters, charts, graphs to have a sound idea as to how you can achieve a concise and on topic well-developed task. 

5-VOCABULARY- Don't learn individual words but learn them in context. This way, you will be able to learn their appropriate usage and score better band. 

WORD LIMIT- Do not forget to attempt the task with the minimum work limit required and legible handwriting. 

Here are a few tips by which you can increase your IELTS writing band score. It doesn't matter whether you've already attempted or you're a beginner, these tips can help anyone who finds it difficult to score in this section. You've to watch a lot of English movies or videos and also read a lot of books. You can mainly learn two things from this, that is structure and linking of the words to the previous paragraph or the line.

You should concentrate on essential aspects like how to structure the essay, how to write a proper introduction because a good introduction sets a pleasant tone of voice which will continue throughout your essay, how to start and end main body paragraphs, and how to write a valid conclusion. As much as the starting is important, so is the outcome.

TO SUMMARIZE:

a. Structure your essay correctly and accurately. Keep your essay simple yet elegant and unique.

b. Focus on the topic you're writing about, don't dilute the essay with irrelevant information. Don't deviate from the subject.

c. Cover all parts of the topic.

d. Explain your position/views/opinion firmly with examples.

e. Use linking words properly; don't overuse them.

f. DO NOT WRITE as a machine would do. Use facts that make sense.

g. DO NOT FIXATE on using only HIGH-LEVEL vocabularies and complex grammar.

First of all,  congratulations on being on band 7. It's an impressive achievement. And of course, you can move from 7 to 8 with a little effort, but that should be very specific. I have to understand in which criteria of assessment or assessment's you faired not so strong. So let me have your sample essay's so that I can help you out in achieving band 8.