Basic Steps:
Download the OBB file(s)
Download the MOD APK
Move the OBB files using a file manager to Android/obb/<packagecode> on your device
Install the downloaded MOD APK
Enjoy
The OBB files are provided either as ".obb" files or as ".zip" files. ZIP files need to be extracted first.
Example:
Let's say the game's package is "com.example.game". The OBB files might be named "main.1523.com.example.game.obb" and "patch.1523.com.example.game.obb" (there can be more than 1 OBB file).
You'll need to place the files "main.1523.com.example.game.obb" and "patch.1523.com.example.game.obb" inside the "Android/obb/com.example.game" folder.
If I don't provide the OBB files in a folder, and the folder doesn't already exist, you'll need to create the "com.example.game" folder in "Android/obb".
Pro Tip:
If you install the MOD APK first and run the game/app, the game/app itself will create the package code folder in "Android/obb". You'll just need to place the OBB files in there afterward.
If you choose this method, be aware that the game/app might crash, hang, or show an error on the first launch because no OBB files have been found yet.
Note:
When the MOD APK requires an update, the OBB files usually need to be updated too. Always make sure to use the latest OBB files. When updating with new OBB files, the old OBB files can be safely deleted.
If you are still having trouble after this guide, please try searching YouTube for "how to install OBB files on Android" or using a Google search.