This is what I did and it got me a welcome, but unexpected 17/20 for my Higher English critical essay, but it was a lot of hard work as I am not a natural at English. I am so glad I never need to study it again!
I started by writing some essay plans. You should be familiar and comfortable with the structure of a critical essay -
Introduction
4-5 paragraphs
Conclusion
Your introduction you can write and memorise most of it for any question you get, and then you only need to adapt a line or two to the question.
For the 4-5 paragraphs each should be structured using PEAR which you should have learned in NAT5.
Pick out some good flexible quotes for theme, 4-6 should do, and write an paragraph plan for each. You then need to learn to take your quotes and analysis and write the PEAR adapting the last part of each paragraph to relate back to the question.
Conclusion, again, write a plan for theme that you can use and adapt it to the question.
Practise a couple of theme questions from past papers using your plan and untimed to get the feel of it and practise at adapting to the question. Hopefully you will be able to use at least 4 of your planned paragraphs in the essay. If not review your paragraphs and perhaps pick a different quote.
Then memorise and practise without the plan.
Repeat for setting and character essays.
Don't use an essay writing service, an prewritten essay is no good to you, you need a flexible plan and to practise adapting your plan to the question you get.