First, you must research the subject to gain a good understanding of it and to support what you will say in the paper. Make sure the resources you use are reputable and that you record sources you found useful so you can return to them. Don't forget to cite all of the sources at the very end of the writing process. Next, make an outline of your paper. Summarise the contents of your introduction, body paragraphs (each with a separate argument for persuasive pieces or a separate sub-topic for expository pieces) and your conclusion. Outlining the contents of your paper before you write it aids organisation. Make sure that the order in which you place the body paragraphs is logical and that it leads your reader clearly to the conclusion you want to them to draw. At this point, you can look at your sources and think about which ones fit where to give the most support for what you're saying.
Now you can start writing the paper itself. Ensure that your introduction gives the reader a clear idea of the specific subject matter you will cover and the sequence of your paper's body paragraphs. Each body paragraph should also start with an indication of which specific part of the overall topic you'll cover, then move on with the details. Your conclusion should leave the reader with a lasting impression, or something to ponder. Essentially, every piece of writing, from a list of instructions to an essay to a novel, tells or implies a story with a beginning, middle and end. Each one of those stories has a purpose or themes. Make sure that your paper does, too, and that this not only becomes clear to the reader after reading your piece, but arouses their interest and remains in their thoughts. That is the trait of a good piece of writing.