Add line numbers using lineno.sty in RevTex 4

1. Download the lineno Latex package.
2. Unpack it (unzip lineno.zip) and copy all lineno/*.sty files into the directory that contains your Latex document.
3. In the Latex preamble include the line
   \RequirePackage{lineno}
   before the \documentclass{...} command.

   Note: this is necessary due to incompatibilities between the lineno package and RevTex. If you are not using RevTex to prepare your document you can also use the standard way: \usepackage{lineno}

4. Line numbers can be activated by adding the line
   \linenumbers
   after \begin{document}