If you already enabled Unknown Sources, skip steps 1,2,3, and 4.

1. Open Settings.
2. Scroll down until you find Security.
3. Tap on Security.
4. Scroll down, then check/tap on Unknown Sources.
5. Download your APK file from Resources if you are going to download Flappy Bird. (If you are going to download another game's APK, then search for it online: [gamename] apk).
6. Go to your File Manager.
7. Check your Downloads folder.
8. Tap on the APK you downloaded.
9. Tap Install.