Come up with a good topic to begin. Look for circuit splits or controversial areas of law. Argue for a change in the law or the way you feel the law should be. In looking for a topic, make sure you search Westlaw and/or Lexis to see if someone else has already written on it. You want an article that is new or one that adds significant material, or a new twist, to an old article. Find something you are interested in, as it will make it easier to write.
Start doing research. This is time-consuming, but you need to find enough material to get you started. Try finding secondary sources first -- other law review articles, CJS, AmJur -- and build on from there with cases.
Start writing. Begin with an introduction if possible. If your law journal has not given you something to go off of, such as a form, check online at other law journals. Many law journals have published student pieces online, giving you a way to get a sense for what you are up against, as well as the proper form to use.
Once you are finished writing your comment, start proofreading, or get another set of eyes to proofread. Go back and double-check your citations. Make sure the content flows naturally and there are topic sentences at the beginning of each paragraph, as well as a coherent theme. Use transitions, and avoid a lot of legal jargon.
Submit your article for review.