In LaTeX documents, users can define custom commands to simplify notation. By using the command \newcommand{\ket}[1]{{\left| {#1} \right>}}, one can easily write quantum state vectors like \ket{X} instead of typing \left| X \right> each time. This approach can also be applied to create similar commands for bra vectors and inner products, such as \bra and \braket, enhancing efficiency in document preparation. This method streamlines the writing process for those frequently using quantum mechanics notation in their LaTeX documents.