Writing macros, defining new commands

New commands can be defined as 

\newcommand{\newtag}{your new command}

 If you would like to pass arguments to the command it is possible as

\newcommand{\newtag}[1]{your new command, your argument is {#1}}

Writing signature in latex documents

Some documents need to be signed by hand, leaving an empty line for the signature and writing the name underneath here is  the solution that worked for me:

First define a new command at the beginning of your document as

\newcommand{\signature}[2][5cm]{
  \begin{tabular}
  {@{}p{#1}@{}}
    \hrule \\[0.05cm]
    \centering{#2}
  \end{tabular}
}

Then use it at the end of the document for instance as

 \flushright\noindent \signature{Firstname Lastname}