In our case we are supposing that the downloaded zip file is 'overt2x.zip'. In your case there would be your theme name instead of 'overt'.
We can use any file archive utility to unpack/unzip our downloaded file. After unzip the file there we can get the folder named 'overt2x'.
Inside this folder('overt2x) you can get other two zip files, please have a look below's file structure.
Please make sure that which moodle version currently you are running.
If the version of moodle you are running is between moodle2.x to moodle 2.3.x, you can do unzip the 'overt2x.zip', Or in case your moodle's version is moodle2.4 or above then you should unzip the folder named 'overt24.zip'.
After unzip anyone of this zip files, you can get other folder named 'overt', that is the folder you'll have to put into theme directory of your moodle for install the theme.
Please have a look below screenshot that shows how to extract suitable theme folder
After get this folder(overt) you can do follow the standard process for install the theme from below link.