For the past seventeen years, I've been writing books and articles based on those books for the antiques and collectibles market ... but not under the pen name you see here. I'll leave it to the sleuthing nature of freelancers to figure out the name I write under for a living. I also edit as part of my job, so I know both perspectives ... the writer eager to get work published and the harried editor looking for the right writer. But you aren't reading this to hear my qualifications, so let's get down to it.
The proposal letter, as the Writer's Market will tell you, needs to be short and sweet. You don't have a lot of space to catch an editor's eye as the editor doesn't have a lot of time to give to each proposal letter. The letter needs to be on a single page and is only three paragraphs long. This letter is written as a business letter in a business format. Place the editor's name, magazine or newsletter's name, and address at the top, followed by the date, and dear editor followed by the editor's name.
Here we need to stop and discuss a few basics. As a new freelancer looking to make a name for yourself, see that name in print, and get paid for your writing (yes you do want to get paid ... eventually ... the freelance business is not about writing for the love of writing alone), you are eager to plunge ahead but you need to slow down and do some basic research first. First, learn about the market you wish to write for. Shelve opinion pieces now ... everybody has an opinion and most people really don't care much about the opinions of a new freelancer as they really aren't useful in the grand scheme of things. Focus on articles that provide people with useful information concerning a field you know about or are passionate enough about to do considerable research. Research the magazines and newsletters that support the market you wish to write for. Look for publications that come out weekly and are small to start with. Save the big publications for when you have made a name for yourself. Right now you want small outfits that need to fill their columns weekly, accept freelancer's work provided it is high quality, and are always hungry for more. Monthly and semi-annual publications can afford to be far choosier.
Read several issues of the publication. This is easier now that some of these publications have online presences, but a trip to the local library or bookstore may still be in order.
Once you've identified the publications you wish to target, have verified that they do accept freelance writing (the book the Writer's Market is very useful in this regard), write to them and ask for a copy of their writer's guidelines. When those guidelines arrive, study them well and follow them to the letter. This is key. If you follow the guidelines provided, you make life easier for the busy editor you are trying to impress as you provide material that requires less work on the editor's part. This always works in your favor.
Okay, now you're ready to consider the proposal letter. There are no nicities to this letter aside from "Dear" and "Most sincerely." The editor is not your friend now and really isn't impressed by freelancers who ask after their health or open with some other timewasting pleasantry. The first paragraph describes what you have to offer. It should begin something like this, "Could I interest you in an article on" followed by the naming of a topic relevant to that publication (trying to sell an article on blogging to a farming magazine will get you nowhere, no matter how stellar your insights) and a working title for the article. Next, tell the editor the article will be so many words long and have so many images (the precise number requested in the writer's guidelines ... see I told you studying those would be useful). After that the first paragraph briefly outlines the article, providing the main topic, and various aspects that support the topic (i.e. an article on a pottery firm's wares would tell where the firm was/is located, provide a brief history of the most relevant information), discuss their wares, and provide examples of their maker's marks). So ends the first paragraph.
The second paragraph provides a working draft of the first paragraph of the actual article. This paragraph is critical. It does not have to be the actual first paragraph complete in all its details. No editor expects that. What it does have to be is the best writing you are capable of producing. The purpose of this paragraph is not so much to give the editor an idea of the opening and direction of the article as it is a sample of your writing to prove you are a professional. It needs to be spell checked, grammar checked, and polished. It needs to be short, sweet, and entirely to the point. Spend some serious time putting together this paragraph.
The third and final paragraph is not as critical, but the promise you provide in that paragraph is extremely important. Begin this paragraph with something like, "If you are interested in this article, I can have the finished copy on your desk in ...". Finish this sentence with a deadline you are absolutely certain you can meet. For me, with my schedule, I was able to promise an article within one month of an editor's approval of the topic based on the proposal letter. As you are new to the field, do not be afraid to set that deadline a good six months away if need be. I cannot stress enough the importance of meeting this deadline. Once an editor agrees to an article and a timeline, that editor has penciled the article in to a specific issue and needs to article to arrive on time to fill the space in the publication.
Remember, in the end, this is not only about getting that first article published. This is about building a professional relationship with an editor. In time, when the two of you have learned to trust each other, that editor may come back to you for material you have not proposed, but material the editor is confident you can provide. When that happens, you will know you have arrived. Follow these guidelines, only submit a proposal for a particular article to one publication at a time, and keep track of what you have sent where. Good luck.