How can I double space a document?

You can use the \doublespacing command provided by the setspace package. The following example uses \doublespacing in the document preamble: 

\documentclass{article}
% Choose a conveniently small page size
\usepackage[paperheight=18cm,paperwidth=14cm,textwidth=12cm]{geometry}
% Load blindtext package for dummy text
\usepackage{blindtext}
% Load the setspace package
\usepackage{setspace}
% Using \doublespacing in the preamble 
% changes the text to double-line spacing
\doublespacing
\begin{document}
\blindtext[1]
\end{document}

 Open this setspace example in Overleaf.

This example produces the following output:

Example of the setspace \doublespacing command

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