You may experience problems if you have never installed manually an Android Game or App on your Android Device so here will be explain in a clean way how to do that.
First of all, you have to ensure your device support the Android game/app you are going to install (you can use the Play Store for this purpose). You have also to ensure your Android Version is high enough to support the game. For example, you can find the following requirement: "Android 4.2 or higher" or "Android 2.3.3 and higher versions of Android Smartphone and Tablets". This means your android version MUST BE higher than 4.2 or 2.3.3! To do this, you need to go to the Settings of your Device, then "Info" and you will find the version of your Smartphone or Tablet.
If your device can run the game/app you are looking for, you can continue reading this guide.
Another important requirement is to allow your device to install "unknown" apps or games. Unknown means that they will not be delivered directly from Google Play or Amazon but they will be downloaded by external links (this website is an example). To do this, go to settings -> security and check "Unknown Resources". This way you will be able to install every apk file you want simply tapping on it.
There are 2 main types of Android Games/Apps installations:
1) SINGLE APK Android Games/Apps;
2) APK + GAME CACHE (or DATA) Android Games/Apps;
SINGLE APK are usually the easiest to install because there is a single file to download, APK + GAME CACHE are a bit harder to install because you usually have to download and install two or more files.
SINGLE APK Android Games or Apps
This is the most common type of Android Games/Apps and its size is usually less than 50 MB. This kind of Game/App doesn't usually have a HD Graphic due to its size so it's suitable for most Smartphones and Tablets because it usually doesn't require an high performing Android device.
The installation of theese files is pretty easy: once you downloaded it, you can move it into every folder of your Android Device and then, with any File Manager, go to the Folder where you moved your .apk File and Tap on it to automatically Install it and you'll be ready to play it in less than 30 secs.
APK + GAME CACHE (or DATA) Android Games/Apps
This type is less common than single APK and it's usually related to Android GAMES and not APPS. The size of theese files can reach up to 5 GB but it's usually less than 500 MB. Theese Games have a higher graphics so they require to have a high-performing Smartphone or Tablet if you want to play it without any LAG or related problems. In some cases, if your device is low-performing, it can happen you can't play the game or it auto-close itself.
The installation of theese games is a bit harder but everyone can do it without problems: once you downloaded the APK and the GAME CACHE (or DATA):
1) Move the .apk file where you want into your Android Device and install it tapping on it (of course using a File Manager to find that file!).
NOTE: Ensure you do not start the game because you first have to move the Game Cache otherwise you will not be able to play it and you'll need to reinstall the game.
2) Extract the Game Cache. It's usually a .ZIP or .RAR file (compressed files, useful because of the smaller size of the file for faster downloads) so you will need to extract it using WinRar or 7Zip or any software you use to extract files.
NOTE: It can happen you will see ".part" files. If so, you have to put all theese files into the same folder, then, you need to extract only the file with the extension ".part1"
3) You will get a Folder containing a ".obb" file when you finish extracting the file (or files). What you need to do is to move the Folder into the following path: sdcard/Android/obb.
NOTE: sdcard is your external memory storage (your SD, or MicroSD), if you do not have an external storage, simply move it into Android/obb.
NOTE #2: it can happen you can't find the "obb" folder. If so, just CREATE the folder into the "Android" folder of your android device.
NOTE #3: sometimes you will need to move the Game Cache to another folder. If so, follow the Installation Instructions provided before you download the game. (ex. sdcard/Android/data or sdcard/Gameloft and so on...)
Now it's time to play! At fist times, you can find this tricky or hard but I can ensure it's very easy to do it!