• The body of the letter.
The body is single spaced.
Ordinarily the body contains more than one paragraph. Avoid both extremely short and very long paragraphs.
You can use either indented paragraphs (in which the first line is indented more than the rest) or block paragraphs (in which all lines begin at the left margin). With block paragraphs you must leave extra white space between paragraphs -- one blank line or one "empty paragraph" is often used, but you can also use Word's extra space before or after paragraph option (Format --> Paragraph...); the extra space should probably not exceed the size of an empty paragraph. With indented paragraphs, extra space between paragraphs is common, but optional. Indented paragraphs should be avoided if the return address was aligned at the left margin.
Special effects like bulleted lists and paragraphs whose left and right edges are both indented should be used very sparingly -- avoid them as much as you can. Likewise, consider whether having your paragraphs fully justified (both left and right edges squared off) will make the letter look too much like a form letter or piece of junk mail.