Simply download the zip and upload the zip (master.zip) under your WordPress dashboard at Appearance > Themes. Or extract and upload via FTP at wp-content/themes/.
You can of course rename the zip file so it isn't called master.zip (you should do this so it makes more sense) and also change the "Theme Name" at the top of the style.css file.
Open up the functions.php file in this folder to add your own customizations, plugins or Wordpress hooks.