Work with a teacher.

You can use spellcheckers (including the MS Word's) automatic proof readers (like Grammarly) to find out your mistakes. Search as: grade level essay checker or paper rater. A lot of sites come up, I don't know which one is good or useful, try some of them if you wish. As IELTS is a British exam choose British sources. This site may help: https://learnenglish.britishcouncil.org/skills/writing/advanced-c1 C1 is a very high level, good luck.

http://www.buowl.boun.edu.tr http://www.buowl.boun.edu.tr/students/fESSAY%20DEVELOPMENT%20EXERCISES.htm I think you may find some tips here. It is a university in Turkey but its courses are in English. It is my alma mater but I graduated a long time ago :) P.S. owl stands for online writing lab

Hi You have to use some structure and complex sentences to get a high grade