Some of my Android apps, such as Die Simulator, Guess The Number and Pong, also come in APK format. If you're a basic Android user and don't know what the heck is APK, I will explain you. APK is an alternative file format used to distribute Android apps outside of Google Play. Below is a video that shows you how to install APK, using Pong as an example:
Note: Once you enable the setting that allows you to install APK files for the first time, it won't ask you to enable it the next times, no matter what app you install(as long as you use the same browser, on some devices it will ask you one time for each app that you use)