The obvious way to do it is to press Enter twice. This is not, however, the
recommended way of adding blank lines between paragraphs because each time
you press Enter you are creating a paragraph, and an empty paragraph (which
contains a multitude of formatting information even if there is no text)
takes up nearly as many bytes as a full one: if half the paragraphs in your
document are empty (to create blank lines between paragraphs), they will
measurably increase the document size.
Instead, add Space Before/After to the paragraph. A quick way to add 12 pts
Space Before (the equivalent of a "blank line" for most body copy) is to
press Ctrl+0 (zero). A better way is to apply a body text style formatted to
have the desired amount of Space Before/After (the built-in Body Text style
by default has 6 pts Space After).
