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You download the APK, then go to a file viewer (I use rar). Allow your app to view your files, then open the downloaded APK with package installer (should be on your phone by default). Allow package installer to add unknown apps to your phone, and download. Now you should have it on you home page.
Note: APKs are Android files, that means they won't work on iPhones.
If your using a computer, download the zip, go to your file viewer, and right click on the file you just downloaded. Click some button that says something along the lines of "decompress zip". Wait for the zip to be decompressed, then you can delete the compressed zip and keep the decompressed zip. Then it's as simple as opening the zip file till you find the actual game and opening it.