Make a plan before you start writing
If you have a solid plan before you start writing you will make sure that you are on topic and that you have an appropriate structure for your essay. It is essential that you respond to the question that is asked. Learn more about planning.

Use sequencing words and connecting phrases
Sequencing words and connecting phrases add cohesion to your writing by showing the relationship between ideas and by sending signals to the reader about your writing. To improve this, refer to my page on Useful Linking Words and Phrases.

Avoid errors with word choices
If you make lots of errors with word choices this makes it more difficult for the examiner to read your writing which lowers coherence. Therefore these errors with word choices lower your score for vocabulary as well as for cohesion and coherence. The negative effect is double!

 Avoid unnecessarily complicated structures and grammar
The more difficult it is for the examiner to follow your writing the lower your score for cohesion and coherence. Just use simple straightforward main points and explain them as clearly and logically as possible. In terms of sentence structure avoid sentences with lots of clauses. I would say a maximum of three clauses. This is because sentences with lots of clauses are hard to read and also if you make any errors with vocabulary or grammar the reader will become totally confused.

Learn how to develop your ideas in paragraphs
When assessing your score for cohesion and coherence the examiner is looking at your ability to structure the whole essay and also your ability to structure individual paragraphs. Paragraphs should focus on one main idea and then that idea should be logically developed in the paragraph through explanation, adding details, and using examples that illustrate the main point.See my model answers to discover how this is done.

From the ebook: Essay Writing Task 2. By Mike Wattie

Available here: https://www.ieltsanswers.com/ielts-books-task2-essays.html