Download a Grav Skeleton from this page (see above list)
Unzip the Grav package onto your desktop
Copy the entire Grav folder into the ‘htdocs’ folder within your MAMP application folder
Launch MAMP
Press the MAMP ‘Start Servers’ button
Enter ‘localhost:8888’ as the URL in your Web Browser
Choose the displayed Grav folder name
Create your site administrator account when prompted
And you’re done! (press the icon in the Admin Panel to preview site)
Now that you have a Grav site present on your computer you can sync those site files to your server in the future, as described in the post Using Grav with GitHub Desktop (hibbittsdesign.org/blog/posts/2015-12-11-using-grav-with-github).