As we know, the essay is called a small essay of free composition, which expresses the individual opinion, thoughts and conclusions of the author on the subject that concerns him. The essay is never intended to be exhaustive, since its purpose is not so much to provide absolutely all the information on the topic or to answer questions, but rather to develop creative thinking and the skill of competent presentation of pro papers reviews.

Essays can vary in several ways:

By content (historical, artistic, philosophical, etc.)
By literary form (notes, letters, reviews, etc.)
By stylistics (criticism, analysis, description, etc.)
By the form of description (subjective and objective)
To write an essay correctly, you need to be guided by its attributes.

These include:

Small volume (there is no limit, but the recommended length is 3-7 pages of printed text).
Specific topic and subjective interpretation (the essay is always written on one specific topic, expresses one opinion or answers one question).
Free composition (there are no formal boundaries).
Uninhibited narration (the manner of communication with the reader should be trustworthy; complex constructions, strict and blurred interpretations should be avoided; the author should be fluent in the topic).
Paradoxes (the essay may arouse surprise in the reader; some authors consider this condition as mandatory).
Meaningful integrity (the essay is characterized by internal coherence and consistency).
Colloquial speech (the meaning should be accessible to everyone, but at the same time templates, clichés, hackneyed phrases, as well as jargon and slang should not be used).
As for the structure, you need to consider two conditions:

All thoughts must be expressed briefly and in thesis form
All theses need to be argued
The outline of the essay is also based on these conditions:

Introduction
Thesis + arguments
Thesis + arguments
Thesis + arguments
Conclusion

When writing an essay, there are a few important nuances to keep in mind as well:

The main topic of the essay and the topics of its sections should be clearly defined
The reader's attention can be captured through interesting facts, allegories, paradoxes, or striking phrases
The introduction and conclusion should focus the reader's attention on the main problem
paragraphs, sections, and red lines must be highlighted
The logical connection between separate sections is necessary
The information should be presented expressively, emotionally and in an artistic form
The more formal rules include:

The presence of a headline
Arbitrary structure (optional, but allowed)
Facts may precede arguments and vice versa
The wording may be identical to the conclusion
No lexical clutter of service details ("I will tell", "I have come to a conclusion", "I have understood", etc.)
The last thing we'll look at when talking about essays are the most common mistakes. Let's list them:

Deficiency of details (any argument needs to be supported by facts).
Protracted introductions (the preface needs to be short and clear).
Long sentences (sentences need to be short and succinct).
Too many words (no need to "pour over" and use words you can easily do without).
Overloaded with official data and scientific concepts (even if they are there, they should be only on the case; the main thoughts should belong to the author).
Poor-quality verification (during verification, you need to eliminate mistakes, unfortunate turns of phrase, etc.; verification should be done at least 2-3 times).
This information should be enough to understand how to write an essay correctly.

Related Resources:

How to Write an Essay: 10 Inaccuracies in Writing an Essay 
How is an essay different from an essay? 
How to write a good essay 
Guidelines for writing an essay