There are only 7 possible essay questions blueprints (agree/disagree, reasons etc. ) IELTS section 2. You should use four paragraph technique to write essays. There should be an introduction, main idea 1, main idea 2 and conclusion. For introduction, you need to write a thesis statement by paraphrasing the question and give a hypothesis (answer) to the problem. The word count should never be lesser than what has been asked. They will cut redundant words. So for 250 word essay, 350 should be good to avoid any penalties. you cannot score high if you don't follow these things even if you have good writing skills.
all what I said is available in this site:
http://yourieltstutor.com/ielts-writing-lessons/
ieltsliz.com
I suggest you to watch IELTS Liz videos (ieltsliz.com) on this. You could even go for her paid videos instead of going to a tutor. She knows exact techniques to crack the exam. I just followed what she showed in her free videos and was manage to get 8 in writing and my spouse managed 8.5 following the same. Since you need more guidance, you could opt for paid lessons.
Similarly, you should follow specific format, salutations etc. for letter writing too.
Avoid simple sentences. Longer complex sentences are good, but use only if you are comfortable with them.
Good luck!