Manually installing apps is easy on any Android device as long as you know of a safe source to get the .APK's (this is the file type of all Android applications). The safest site to get them is from APKMirror. Keep in mind, this site only hosts safe APK's and they are only for apps that do not cost any money. You will not find paid applications on this site and I discourage you from looking elsewhere for them.
First, ensure that you are able to install applications from Unknown sources. This can be done by simply going to your device settings, tapping 'Security', and flipping the switch for 'Unknown sources'. This will allow you to install applications that were not downloaded directly through the Play Store.
Second, download the latest version of Pokémon GO from APKMirror at the following link from your device:
http://www.apkmirror.com/apk/niantic-inc/pokemon-go/
Once this has finished downloading, simply click the notification and then choose 'Install' on the dialog that pops up. It will take a moment for the app to install, but once it is done you will be able to play Pokémon GO!
Third, be sure to go back into your 'Security' settings and turn 'Unknown sources' back off so that you do not accidentally install an app that you did not mean to install.
If you do not feel comfortable with this method, check back at the official Pokémon GO website from time to time to see if your country has been added. You can also add Pokémon GO to your Wishlist in the Play Store and check there to see if you're able to install it.