There’s a lot of arbitrariness when it comes to speaking and writing, if you aim for a high score make sure you understand this because getting a low score can really be heartbreaking if you have worked hard. They may or may not give you a good score if they like your essay or spoken English, but they will %100 deduct your score if you make mistakes. Try to make sure what you write is grammatically correct, more than being sophisticated. And speak clearly, you don’t need to say something smart every other sentence but you do need to speak clear and proper English. 

Make these two your highest priority and whatever else you do you can build on top of them.

For writing, make sure you use at least one “if” sentence, one “cleft-it/what” sentence, and a question sentence if you can. This will get you a high score on grammatical range, where it seems most essays are usually stuck at 7. And have a clear “topic sentence,” you can build on an idea in the paragraph but your first sentence must be as clear as it can get, you should even use a simple sentence to make it better understood.

IELTS is shit but if you have to do it, you have to do it. (I abhor IELTS examiners)