How to Install APK files
Firstly download the APK file from either of the links given below.
If you get a prompt as shown in the picture, Tap OK. (I assure you Loona Stickers will do no harm to your phone)
Next open any File Manager app and navigate to the downloaded APK file (Usually the Downloads folder)
Tap on the APK file and tap INSTALL
If you get a similar prompt from your file manager, Tap on Settings,
(You may get a different prompt, scroll down to know more)
Turn on Allow for your File Manager App
(You can come back to your settings after installing and turn it back off just in case you're worried)
After allowing the required setting, you can go back and once again Tap on the APK file to proceed with the installation.
If you get a prompt like this one, Tap on Settings,
Enable the Unknown Sources option.
(You can come back to your settings after installing and turn it back off just in case you're worried. You can find this option in your Settings → Security → Unknown Sources and turn it back off)
After allowing the required setting, you can go back and once again Tap on the APK file to proceed with the installation.