Format your pitch properly. This means sticking to one page of quality white or ivory paper with 1 inch margins on all sides, single spacing throughout, using size 12 Courier New size or another uncondensed font, creating a simple header with your name, address, phone number and email and enclosing a self-addressed stamped envelope with your pitch so that it can be returned to you at no cost.
Give her the title of your work, its projected length and its genre in your first paragraph; then throw out your "dramatic hook," the part of your story's premise that is most likely to catch her attention. Consider mentioning if you have met or corresponded with the editor previously -- personal connections are always helpful in publishing.
Summarize your story or novel using only four sentences in paragraph two. This will form the crux of your pitch: You must compress your narrative's premise, problem, action and conclusion into four straightforward, unambiguous sentences. Do not hold anything back from the editor that you normally would from the reader -- she gets to "know the answers," and this is her cheat sheet.
Deliver a concise biography of yourself as an author in paragraph three. Stick only to information relevant to your work -- the editor does not need to know where you went to elementary school, but she may wish to know where you earned your MFA, for instance. Let her know about previous publications and any prizes or awards you have won for your fiction.
Thank the editor for her time in your fourth and last paragraph. Mention that you hope to hear from her soon.