I want to be able to customize my header/footer so far that I can change the rules that separate it from the document. Look for example at http://ww1.microchip.com/downloads/en/appnotes/00734b.pdf (second page and further), I want to be able to make header/footer rules like that.

Currently I'm using the fancyhdr package, but that's not necessary.
I already tried to add \hrule in the footer, and that's working, but I'd think there is a neater solution?

The code I used for that:

\renewcommand{\footrulewidth}{0pt}
\getfile\vskip2pt
\hrule width \hsize height 1pt\vskip1pt\hrule width \hsize height 2pt %
}
\fancyfoot[L]{
\hrule width \hsize height 2pt\vskip1pt\hrule width \hsize height 1pt %
\vskip2pt Some text.
}
\fancyfoot[C]{\vskip5pt Some text.}
\fancyfoot[R]{\vskip5pt Some text.}


To replace the rules of fancyhdr, you can redefine \headrule and \footrule including just the solution at your question Two rules directly under each other

\documentclass{article}

\usepackage{lastpage} % number of last page
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{}

\renewcommand\footrule{\begin{minipage}{1\textwidth}
\hrule width \hsize height 2pt \kern 1mm \hrule width \hsize
\end{minipage}\par}%

\begin{minipage}{1\textwidth}
\hrule width \hsize \kern 1mm \hrule width \hsize height 2pt
\end{minipage}}%

\lfoot{\today}
\rfoot{Page \thepage\ of \pageref{LastPage}}

\begin{document}
\section{Test}
\end{document}


I'm not going to go into great detail as per your requirements, as this topic is covered (in some variant) in many other posts, however, basically, this is the way that you do it. You need to modify the format of the fancypagestyle, then when that style is called, the changes will be reflected in the document

\usepackage{fancyhdr}
\fancypagestyle{plain}{
\fancyhf{} %Clear Everything.
\fancyfoot[C]{\thepage} %Page Number
\renewcommand{\headrulewidth}{1pt} %0pt for no rule, 2pt thicker etc...
\renewcommand{\footrulewidth}{1pt}
\fancyfoot[L]{BOTTOM LEFT}
\fancyfoot[R]{BOTTOM RIGHT}
}

\pagestyle{plain}
\begin{document}
%Document Content.

\end{document}


For the double lines, you can do something like this.

\fancypagestyle{plain}{
\fancyhf{} %Clear Everything.
\fancyfoot[C]{\thepage} %Page Number
\renewcommand{\headrule}{\hrule height 2pt \vspace{1mm}\hrule height 1pt}
\renewcommand{\footrulewidth}{1pt}
\fancyfoot[L]{BOTTOM LEFT}
\fancyfoot[R]{BOTTOM RIGHT}