There should be clear, roughly equal margins on all four sides. Do not use a single hard-return in the midst of a paragraph block to suggest distinctions between 'major' and 'minor' thoughts. Each paragraph should be a coherent, logical entity which is separated by double spacing from the paragraphs above and below. The text body should normally use a size-12 Times or Arial font. Unless otherwise specified, papers should be single-spaced.
Speeches are usually triple-spaced, often with a larger font for ease of reading.
After a comma or semicolon, there was one space before the next character. After a colon there were two spaces. One should still follow these practices. When using proportional fonts, the font itself will adjust the two literal spaces to one proportional space, but leave the two literal spaces intact in case the text is later converted to a monospace font. The quick . . .