The standard way to do double spacing which everyone recommends as clean and neat is to use the setspace package since it doesn’t double-space your footnotes which is generally desired.

Turns out that (a) this isn’t the typographical standard of double-spacing (b) it’s not the same as Microsoft Word which everyone else’s extended essays/dissertations/theses will be written in.

So instead use this:

\usepackage[doublespacing]{setspace}
\setstretch{2}

and if you change the spacing be sure to do it again:

\singlespacing
[...]
\doublespacing\setstretch{2}

(source)