First things first, if you haven't yet submitted to an online publication you may need to know a few things about submission formatting. If you've already submitted online several times, you can skip this.
Online Submission v Submitting for Online
The first thing you need to know is, online submission is not necessarily the same as submitting for online. That is, many print publications now have some form of online submission process or accept submissions in the body of e-mails or as attachments. However, unless guidelines say otherwise, you can assume that print publications want pieces formatted for print. That generally means one inch margins all around, double-spaced, and new paragraphs signified by indented first lines.
Online Submission Guidelines
Formatting for online publications is totally different, but first, lets talk about the submission process itself. Obviously you can assume that an exclusively online publication is not going to ask you to submit via snail mail. However, there can be a range of differences in how they ask you to submit, and you need to check guidelines carefully so your submission will not be automatically disqualified.
Many publications like the one I currently work with, Sotto Voce, have an online submission system where you can either upload or copy and paste your submission into their system. With some systems you may receive an e-mail notification and a registration number you can use to check the status of your submissions. Others will just provide a window that says your submission was received. Online submission systems can be a little scary the first time through, but if you follow instructions carefully, you should do just fine. Many also allow you to view your submission to make sure it looks right and no strange hieroglyphics show up in the text.
There are still some publications, like The Rose & Thorn that ask for e-mail submissions. When submitting via e-mail it is extremely important to check guidelines as to whether submissions should go in the body of the e-mail or be sent as an attachment. In either case, not following the guidelines will usually lead to automatic disqualification, and under the circumstances the publication may not feel constrained to explain. Consequently you may find yourself waiting for a response that will never come.
If you've ever seen how an improperly formatted submission appears online, you will take great pains to ensure that your submission is formatted correctly. For older writers one of the hardest things to get used to is not adding that extra space after periods. That's a hold-over from block printing days––or something like that. It really doesn't apply anywhere anymore, but it is especially annoying with online submissions because it takes up space and looks strange. Remember, only one space after a period as well as a comma.
The standard format for online submissions is what you see right here in this post. That is, single-spaced, with double-spacing between paragraphs, and no indented lines. Another very important point to remember is no hard returns except at the ends of paragraphs. A "hard return" is when you hit the return or old enter key at the end of a line, rather than letting your word processing program wrap it automatically. This is not usually a problem for those who've never typed on anything but a computer or didn't work with old word processing programs. If you learned to type on a typewriter, you may get the urge to divide your lines yourself or hyphenate words that don't fit completely on a line. (One of the many wonders of the modern age is no longer having to know where the syllable break is.)
Include Proper Information and Never Be Sloppy
The final part of submitting to online publications is really a catch-all. Whether submitting for print or for online, for e-mail submissions always check if a special subject line is required. If not, keep it brief and always include the word "Submission." You want to avoid SPAM filters at all costs. As an editor I have always appreciated knowing whether it is a fiction or nonfiction submission as well, because it can be hard to tell.
Always make sure you include in your e-mail all information required in the guidelines. This might include full name, e-mail address, and/or a brief bio. When nothing is specified always treat your submission e-mail as you would a cover letter. Try to use the editor's name in your greeting, if possible. Make it concise but include any other publishing credits or awards that might grab the editor's attention.
Some online submission systems have a place for comments. If so, this is the place for your brief bio and credits.
Finally, don't make the mistake of thinking that online submitting is less formal and requires less care than print. Even if your submission is pristine a sloppy introductory e-mail is like walking into a job interview with a rumpled suit. And here's an insider's tip. At Sotto Voce and most other publications, clean copy is one of the criteria for acceptance. If you're bad at proofing your own work, as I am (I bet I'll find all kinds of errors in this quick post), then ask a friend to proof your work for you.
First things first. Proper formatting and good proofing won't guarantee submissions, but improper formatting will definitely disqualify your submission.