Choose a topic for your essay that people disagree on, such as health care reform, the best age to get married or whether cats or dogs make better pets. Finding a topic you feel strongly and are knowledgeable about will make writing easier.
Research your topic. Do a search online or visit a library to make sure enough sources exist to support your opinion. Study the reasons presented for and against your position. An opinion essay doesn't require research, but your argument will be stronger if you can find reliable sources -- such as expert opinions, government websites or scholarly journals -- that agree with you.
Formulate your thesis, the sentence appearing in your essay's introduction that states your main point. A good thesis is specific, easy to understand, written as a statement rather than question and narrow enough to cover in the length of your essay.
Create an outline for your essay. Identify three or four reasons supporting your thesis to use as the main topics in the body of your essay. Also choose one opposing point you will respond to. Your essay will be stronger if you demonstrate that you are aware of others' objections and have answers for them.
Write your essay. Start with an introduction that piques the reader's interest and states your thesis. Develop each of your supporting points in two or more paragraphs, drawing on your research and your own ideas. After you have made a solid argument, briefly mention one opposing point. Either refute the point and show why it is wrong or concede that it is partially correct but explain why it doesn't discredit your opinion. Sum up the essay with a conclusion that restates your thesis and main ideas.
Revise your essay to check for awkward sentences, inaccurate statements and errors in punctuation and spelling. Mistakes can easily creep in as you write so you need to read through your essay several times to make sure it effectively expresses your ideas.