These instructions are meant to help you on your way if README.md still leaves some gaps.
If you are comfortable with a pip install you should not read on ;-)
The preferred way to install packages on your system should be to use some form of package manager like APT for Debian GNU/Linux and derivatives or DNF for the Red Hat families, or the Windows Store on Microsoft Windows.
eml2pdf is far from ready to be included in vetted package distributions such as the official Debian packages as this requires a lot of packaging work on top of "just delivering a working script".
This is why we currently release eml2pdf via PyPi, the Python Package Index. Next steps on the roadmap for eml2pdf will be to release packages and packaged executables next to PyPi and the source packages.
The easiest way to install eml2pdf is to install it via pip in a virtual environment. Pip is the Python package installer. You can find general info on the Python documentation pages on installing python modules.
On Linux systems you should have python3 available. On Debian you would need to install the python3-venv package to create and use virtual environments. On Windows you can find Python on the Windows Store and some help on Python on Windows for beginners from Microsoft.
Next you should create a virtual environment using
python3 -m venv <path_to_your_venv_location>. Basically, virtual environments
create a separate directory with an independent set of Python packages without
polluting your Linux distribution packages. Refer to
the Python documentation on virtual environments
for more info on virtual environments.
Activate your venv using source <venv>/bin/activate on Linux or execute
<venv>\Scripts\Activate.ps1 in PowerShell on Windows if you are working on
Windows with python3 in the same environment where you have Pango available.
Refer to information below for Windows.
In your shell, you can then issue pip install eml2pdf to install the
latest release from PyPI, the Python Package Index, or
pip install <path_to_source_dir>, which will install the git or other source
pacakge you downloaded and unpacked in the virtual environment.
The eml2pdf command should now be available in your shell if you activate your venv.
Using eml2pdf on Windows should be possible, but it's not easy due to the Pango dependency. Refer to weasyprint install instructions for Windows and then follow the pip installation steps above.
We'll try to make that easier if we get some requests for a packaged Windows executable.