Once you have selected the process you want to write about, outline your main points. If the process includes a sequence of steps, outline those steps on a notepad or in a word processing program. Order the sequence correctly on your outline and develop subpoints that you will use to thoroughly explain the process to your readers. Decide whether you want to explain, interpret or communicate the significance of the process in your essay. Select sources that you can reference to support your main points and process steps. Write your essay and edit it for grammar, spelling and punctuation and to make sure that you've covered every point on your outline.
Think of a process that you know better than most people. For example, canning peppers, baking a cake, planting a garden and repairing a roof are all processes that you can write about in an essay. Those are also examples of hands-on processes, or processes that require more than mental effort to complete.
Think of a process about which you know a lot. Start by considering things you've done many times, such as writing papers or researching topics. Then narrow your focus to write an essay that details the steps necessary for performing a detailed aspect of writing or researching. For example, you could write an essay about how to find a good book, or how to use the catalog system at your library. You can also frame your essay as a "how not" paper. For example, you could write about how not to write a bibliography or how not to write an interesting introduction.
If you've recently held a summer job or have a parent or relative in an industry that interests you, you could write your essay about an institutional process. Your essay could describe processes such as how fish sticks are made or how Burger King hires employees or how oil gets to U.S. gas stations.