\documentclass{article}

% Set page size and margins
% Replace `letterpaper' with `a4paper' for UK/EU standard size
\usepackage[letterpaper,top=2cm,bottom=2cm,left=3cm,right=3cm,marginparwidth=1.75cm]{geometry}

% Useful packages
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage[colorlinks=true, allcolors=blue]{hyperref}

\usepackage{natbib}
\bibliographystyle{abbrvnat}
\setcitestyle{authoryear,open={(},close={)}} %Citation-related commands

\title{Your Paper}
\author{You}

\begin{document}
\maketitle

\begin{abstract}
This is a modification of a help file that Overleaf provides. 
 You can find more examples \href{https://www.overleaf.com/latex/learn}{here} or \href{https://www.overleaf.com/latex/examples}{here.}
 \end{abstract}

\section{Introduction}

\section{Data}

\subsection{How to add Citations and a References List}

The ibliographic information is in the file \verb|sample.bib|.  You can download entries from, for example NASA ADS, or there are tons of online generators, \href{https://url-to-bibtex.vercel.app/}{for example.}


You can then cite entries from it, like this: \citep{greenwade93} or \citet{greenwade93}. 

If {\it not} using NatBib, remember to specify a bibliography style.  You must always specify the filename of the \verb|.bib|. You can find a \href{https://www.overleaf.com/help/97-how-to-include-a-bibliography-using-bibtex}{video tutorial here} to learn more about BibTeX.

%\bibliographystyle{abbrv}
\bibliography{sample.bib}

\end{document}