The examiners are looking at your ability to use English effectively.
This involves grammatical correctness:
The use of clear sentences.
Appropriate tenses of verbs – a sentence should not include a mixture of present and past tenses. Verb and subject agreement must be respected i.e. a singular verb should have a singular subject.
Do not use extended compound sentences using lots of coordinate conjunctions (and or but). Often a simple sentence is more expressive than one which continues for many lines.
The way you express yourself is more important than what you say. The examiners are not marking your opinions but the way in which you express them.
Vocabulary is the discriminating factor between a six and a seven. It is in your own interests to develop your vocabulary. When you encounter a new word (or an old word used in a new way) write it down with the meaning. Try to learn 10 new words a day, and make a point of using them in your writing.
Paragraphs should have clear topic sentences. The ideas in a paragraph should be linked to that topic sentence.
Your introduction should state a simple proposition not go on to elaborate all the details of future paragraphs.
Be very careful that your conclusion is a representation of all your evidence. If your conclusion contradicts your introduction the examiners will be unimpressed by your logic.
Do not write massive, long paragraphs with rambling ideas. The examiners will appreciate your succinctness.
Vary the opening of each paragraph. You need a variety of beginnings which will keep the reader interested and entertained. Practise using these openings so that you can use them in a refreshing manner.
A good example is a worthwhile means of illustrating an idea, but don’t let that example take over the essay. Keep it brief and relevant.