That's why I referred you to Shauna Kelly's tutorial. It explains. The single spacing, itself is not bad, but you should leave the automatic spacing between paragraphs. Single spacing is harder to read, though, than the slightly expanded spacing that is the default.



In particular:
2.1 Why you should not press Enter at the end of every line
2.2 Why you should press Enter only once to end a paragraph
3. Using Styles to Format Text



I really recommend that you start at the beginning. You'll be glad you did.