Unity: An essay, written on a particular topic, should have uniformity. All the ideas and thoughts expressed in the essay should be related to the topic. Similarly each paragraph should have unity. Unity means that each paragraph should be about ONLY ONE idea. Different paragraphs have different ideas relating to the topic but each paragraph is confined to one idea. Never embed a different idea in a paragraph which explains a particular idea.
Coherence and Order: The ideas should be expressed in a logical continuity. Coherence means that ideas flow smoothly from one sentence to the next sentence. Order is the logical sequence of ideas presented in the essay. Order of ideas means what should be written first and what should be written next. Ideas should be expressed in a proper order of importance or events.
Avoid repetition of a same idea. Don’t reproduce a same idea again and again which you have already narrated once in your essay. Repetition reveals a very bad impression on the reader. Similarly it is better to use different words (noun, verbs etc) for a same meaning if it is repeated again and again.
Avoid unnecessary details and explanations. Always be focused on the subject matter and try to write to the point. Don’t try to elongate your explanation by giving unnecessary and irrelevant details. Your explanation may be neither too long nor too short but should be comprehensive and sufficient in knowledge about the topic.
The way of presenting ideas and thoughts should be appropriate. The way you describe, explain, elaborate or discuss about an idea counts a lot for writing a good essay. Always try to write in a simple, clear and easily comprehendible manner. Never write ideas in a convoluted manner. Write short and simple sentences instead writing very long sentences containing many clauses. Writing on different dimensions of the subject matter should be in a way that each aspect should be fully unveiled so that there may be no room for ambiguity for the reader.
Proficiency and competency in writing in English
It is an open secret that writing standard English and grammatical accuracy play a vital role in success in essay writing for CSS Exam. You should know the art of literary and scholarly writing. It needs a long discussion to expatiate about improving written English but let me give you some rudimentary tips. A basic attribute which is to learn idiomatic usage in writing an essay should be developed. For earning grammatical accuracy in your writing, study English grammar thoroughly (not cursorily) to learn the correct usage of grammatical units i.e. verbs, adverbs, adjective, coordination, subordination, subject-verb agreement, phrasal verbs etc.