You didn't say whether you're attaching it or pasting it into an email. If you're attaching it, do it the way they suggest in From Keyboard. If you're pasting it, just do what makes sense. There really aren't any rules. Single spacing is fine and just put the word count wherever it's convenient, maybe under your heading at the top. They can do a quick word count themselves or reformat it to double spacing. Here's how I do it (and yes I've sold a few):
Picture book submission (title: word count)
Single spaced manuscript.