\documentclass{article}
\usepackage{charter} % Use the Charter font
\usepackage[
	a4paper,    % Paper size
	top=1in,    % Top margin
	bottom=1in, % Bottom margin
	left=1in,   % Left margin
	right=1in,  % Right margin
	%showframe  % Uncomment to show frames around the margins for debugging purposes
]{geometry}
\setlength{\parindent}{0pt}     % Paragraph indentation 
\setlength{\parskip}{1em}       % Vertical space between paragraphs
\usepackage{wallpaper}
\usepackage{mdframed}
\usepackage{graphicx}       % Required for including images
\usepackage{fancyhdr}       % Required for customizing headers and footers
\fancypagestyle{firstpage}{%
	\fancyhf{} % Clear default headers/footers
	\renewcommand{\headrulewidth}{0pt} % No header rule
	\renewcommand{\footrulewidth}{1pt} % Footer rule thickness
}
\fancypagestyle{subsequentpages}{%
	\fancyhf{} % Clear default headers/footers
	\renewcommand{\headrulewidth}{1pt} % Header rule thickness
	\renewcommand{\footrulewidth}{1pt} % Footer rule thickness
}
\AtBeginDocument{\thispagestyle{firstpage}} % Use the first page headers/footers style on the first page
\pagestyle{subsequentpages} % Use the subsequent pages headers/footers style on subsequent pages
%----------------------------------------------------------------------------------------
\begin{document}
%%%   set the background of letter   %%%
\newcommand\BackgroundPicture{
   \put(0,0){
     \parbox[b][\paperheight]{\paperwidth}{
       \vfill
		%\node[opacity=.06]
       \centering\includegraphics[width=0.3\paperwidth,height=0.3\paperheight,keepaspectratio]{background.png}
       \vfill
     }}}
\AddToShipoutPicture{\BackgroundPicture}
%%%   the basic information of your letter   %%%
\includegraphics[width=0.5\textwidth]{HKUST.png} % Logo
\hfill
\begin{tabular}{l @{}}
	Date: \today\\ % Date
	Name: abc \\
	Phone: 00000000\\
	Email: @connect.ust.hk
\end{tabular}
\vspace{-1em} % Pull the rule closer to the logo
\rule{\linewidth}{1pt} % Horizontal rule
\bigskip\bigskip % Vertical whitespace
%%%   The body of the letter   %%%
\begin{tabular}{@{} l}
	\textbf{Mr./Mrs.\ name} \\
	\textbf{Title of Letter} \\
	\textbf{The name of institute / department / school / company / ...} \\
\end{tabular}
\bigskip % Vertical whitespace
Dear Mr./Mrs.\ name,
\bigskip % Vertical whitespace
Greet Sentences......
Content......
Some points need to stress respectively:
\begin{itemize}
\item ......
\item ......
\item ......
\item ......
\end{itemize}
Summary......
\bigskip % Vertical whitespace
Best Wishes,
\includegraphics[height = 1cm]{signature.png}
Your Name % If a signature exists, do not type your name hear.
Department of ......
The Hong Kong University of Science and Technology
\end{document}