Here's how to do it:
Setting up your device
From your smartphone or tablet running Android 4.0 or higher, go to Settings, scroll down to Security, and select Unknown sources. Selecting this option will allow you to install apps outside of the Google Play store. Depending on your device, you can also choose to be warned before installing harmful apps. This can be enabled by selecting the Verify apps option in the Security settings.
On devices running an earlier version of Android, go to Settings, open the Applications option, select Unknown sources, and click OK on the popup alert.
The next step will be finding an Android package file, also known as an APK, which is the way Android apps are distributed and installed.
You can either download the APK file on your mobile device or on your computer, although the latter is a little more difficult. To get started, download an APK file using either Google Chrome or the stock Android browser. Next, go to your app drawer and click Downloads; here you will find the file you just downloaded. Open the file and install the app.
If you downloaded the APK file on your computer, the process is slightly different. You must connect your Android device to the PC and enable USB mass-storage mode. The next step is to drag and drop the file onto your device. Then, using a file manager, such as Astro or ES File Explorer, you can locate the file on your device and install it.