Always use white paper and black ink.
Always double space.
Always type or print out on one side of the page.
If using a word processor, print out manuscripts in 12 point Courier. Text should be left justified (the title and your byline can be centered). Don't use fancy, fonts or creative formatting. This annoys editors. Don't include illustrations.
Put your name, mailing address, phone number and social security number in the top left-hand corner of the first page.
Also put the word count in the upper right-hand corner. Center your title and byline halfway down the page and start the double-spaced text of your story after that.
On each succeeding page, put your last name, the title (or if it's long, just the first word or two) of the work and the page number at the top right of the page.
Underline anything you want to appear in italics. Don't actually format the text in italics or boldface (I know it's maddening not to use all that fancy stuff your word processor will do, but this is the proper format).
Here's how to count words in a manuscript. First off, don't rely on your word processor's count, because those vary. Publishers count 6 units as a word, and it makes no difference whether those units are letters or blank spaces. So to get the right approximation, all you have to do is:
1)Count ALL the spaces between your margins and divide by 6. My spaces happen to come to 66; so when I divide, I get 11 words per line, even if there's only one or two "real" words on that line and the rest is blank.
2) Count the number of lines per page and multiply by words per line (in my case, 11). That, amazingly enough, gives you the count for that page (again, making no difference how many "real" words on are the page).
3) Multiply that count by the number of pages in your story or novel, round to the nearest 100, and you have the word count the editors are looking for. List this in the upper right-hand corner of your manuscript thusly: About 12,500 words.
Learn manuscript submission etiquette
Editors are almost always overworked and underpaid, but they usually manage to be nice people in spite of that. Don't get on an editor's bad side, as it can adversely affect your chances of selling him or her a story (coincidentally enough). The following suggestions will help you stay on an editor's good side.
Paperclip your manuscript pages together and put it in a plain 9" x 12" manila envelope. ALWAYS include a self-addressed envelope with sufficient return postage. This is essential.
Don't try to "catch an editor's attention" with flashy paper/envelope/ink colors. Don't send gifts or "surprises" like adding that glitter stuff that spills out of the envelope when it's opened.
If you include a cover letter, keep it brief and businesslike. Don't brag about how great the story is, or try to "explain" the story, or give a blow-by-blow account of how you wrote it, or anything like that. All an editor wants to do is read your story, and the story will have to stand on its own merits.
If, alas, your story should be rejected, by all means don't send a letter to the editor complaining about it. After all, you'll probably want to send that editor another story sometime.