I just wrote about how you use the custom fonts on your iPhone or iPad, but for the Mac, things are a little simpler:

1. First figure out what font you want with What Font Is? or similar utility.
2. Then see if that font is available in Homebrew Fonts because that makes it much easier to manage the installation, the thing you need to do is brew tap homebrew/cask-fonts and then you can look at all the fonts they have with brew search font-the_name_your_font
3. When you find it then the installation is a simple brew install font-abel for instance to install the Abel font.

If the font doesn't exist, then it's a little more complicated: