There are two components to processing your document, the PreTeXt stylesheets and the xsltproc program. We work at the command-line inside of a terminal or console. If you do not know what this is, it will seem very primitive at first. Sometimes the old ways are the best ways. This will be called a “Command Prompt” in Windows or a “Terminal” on a Mac. In Linux it may be known as a “console” or a “shell”. A tutorial, which is Linux-specific, can be found at Ryan's Tutorials and certainly others exist.
The operating system on a Mac is built on Unix, which is very similar to Linux, so most of the directions here will be little changed between the two. Procedures can be very different in Windows (Appendix E, Appendix F). One alternative is CoCalc which provides a full Linux computer for free in your web browser, so that may be an excellent place for initial experiments (Section 4.12).
Step 1: PreTeXt
You need to obtain the PreTeXt stylesheets, which are the main part of PreTeXt. Since you are reading this, it may be possible that you have this already. You can use git to clone the PreTeXt from the GitHub repository, and then be sure to checkout the dev branch to have the latest version. This is the best way to go, and you should only download the repository as a zip file once for an initial experiment, and then switch to using a clone instead.
Once you have a clone of the repository, you can issue git pull, and git will update your local copy with any recent changes. You should do this regularly—meaning on the order of daily. See the FAQ entry for more about why we expect you to do this.
See the website at mathbook.pugetsound.edu for details and commands for this step, right on the main page.
Step 2: xsltproc
This is the command-line program which takes your document and a PreTeXt stylesheet to together produce output. On Linux or a Mac you probably already have it installed as part of system software. On Windows it is not so simple.
In either case see the website for details abut verifying you have this, or how to install it.