Well, first things first: double spacing is NOT two spaces between every word, or sentence.
Double spacing means having a blank line between each line of text.
The image here shows how to set it up in Microsoft Word, although many other word processors operate in a similar fashion. In Word, go up to Tools, select Paragraph and the following window will appear.
About half way down the window are the options for spacing. In the middle of this section is the drop-down menu for line spacing. Select the arrow, and then choose Double from the drop down list.
THAT'S IT! Yes, that is all there is to it. (Okay, I lie, you also have to press OK at the bottom, too.)
DO NOT select anything in the boxes to the left, labelled Before or After. These need to be left at 0pt. (I've explained why in last week's post - I'll Put That Bit There ... Part 2)
So, why do we use double-spacing? Basically, because it's tradition. It's what writers always have done, since the days when anything printed, be it newspaper, magazine, or book, was published using hot metal presses. An editor would take a double-spaced typescript, use the extra space between each sentence to annotate to the typesetter any changes that needed making, or inserting any special instructions to the typesetter about headings, or inserting images, and then send the document to the typesetter for setting out on the metal presses.
Proofreaders and copy-editors needed double-spaced text to give them the space they required to annotate any corrections.
But since the advent of computers, hot metal presses have not been used to publish material. So why do we still do it? Double-spaced text is easier to read. (Try it. Print out one of your typescripts in double-spaced format and then print out the same text in single spaced format. Which is easier on the eye?) This is why writing competitions ask for double-spaced text. It is far easier for the judge to read. I once had to judge a pile of 166 short stories (of up to 4,500 words each) and it's surprising how quickly the eyes tire.
Editors know how much text there is on a double-spaced page. And yes, the gaps still give the editor space to write notes or comments for other staff to action.
When should you not double-space your text? When the editor tells you there's no need to. Yes, that's right. If an editor tells you there's no need to double-space, then you don't have to do it. But don't do this until you have permission from the editor. (Let's be honest, if an editor says he wants your manuscript on pink paper, in Comic Sans font, at size 8, then that's what you bloody well give him!) But until you are told otherwise, you give an editor double-spaced text.
So, when you set up your article, short story or book template, make sure you include double-spaced text. Whilst double-spaced text is no longer required for the publication process, it's what writers have been doing for years and what many publishers continue to ask for today.