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)