Before writing an essay introduction, make yourself ready in a preparatory writing phase: concentrate, brainstorm and collect ideas. The very fact you have your assignment means that you already have stored some specific knowledge on the topic. Try to design the main conception of essay on top of your head without digressing on additional sources (moreover, it is maybe the case when you are not allowed to check with literature, as on writing an expository essay in class).
At the second step, you should put your pen to paper and arrange your thoughts on the draft, being guided with style requirements. Since you discuss some fact, event or people’s opinion, but not your personal point of view, address your reader in the third person and never write in the first person, which will be considered as a mistake any time you are writing an expository essay. Start your introduction with a topic sentence, which firmly states the thesis, remember not to employ your position while expounding the thesis. Your body paragraphs ought to cover issues that develop the stated thesis - whereas your designed outline in its compactness and brevity should be next to a small business plan! In that way, evidential paragraphs might be taken as little representatives for every branch of this plan.
The third step in writing an expository essay is a revision stage - here you check your work with the initial aim and decide whether the topic has been revealed in accordance to the requirements of your assignment. Also, try to think of another way your work may have been written. Something like: "Who can write my paper for me and what would they say?" Does this imaginary essay conception differ from your conception qualitatively? Why?
The fourth step is the most technically difficult, say, not only in writing a expository essay, but in any other essay as well. It is time for your proofreading and correction of semantic, lexical and grammar mistakes. Be careful and attentive, read through it at least several times.
Ultimately, on the fifth stage together with your tutor or a friend, who is a talented critic, test an accomplished work on an imaginary audience. The outside objective view on your essay is but inevitably important provided that nothing on Earth could spoil your writer rapture faster. But do not dread a possibility to be severely criticized. While writing an expository essay or any other, it is always better to stay on real ground and hence to avoid nurturing some empty expectancies.