To use these macros in your project, create a symbolic link to tex-macros in the same directory as your main .tex file:

ln -s PATH/TO/REPO/tex-macros ./tex-macros

Then you can \input any of the files, e.g.

\input{tex-macros/article-preamble}
\begin{document}
...
\end{document}

The tikz figures are designed to be either compiled as standalone files or included with \include, with an optional global variable \scale defined, e.g. \def\scale{1.5}. This is automatically done by a locally defined function \includetikz, used like so:

\includetikz[scale=0.8]{poseidon}

It will automatically find poseidon.tex inside of tex-macros/tikz-figures.