If you don't already have Python 3+ installed, grab it from https://www.python.org/downloads/. You'll want to download and install the latest version of Python 3.x. As of 2019-10-14, that is Version 3.8.
On the repository page, click the green "Clone or Download" button at the top right of the page. If you want to get started with this script more quickly, click the "Download ZIP" button, and extract the ZIP somewhere on your computer.
Or use git if you know what you are doing.
In a command prompt or Terminal window, navigate to the directory containing this repository's files. Then, type the following, and press enter:
pip install -r requirements.txt