Hello! It's simple. First, download the Minecraft APK file from a trusted site, for example, https://minecrftapk.org . Go to your phone settings, to the "Security" or "Applications" section, and allow installation from unknown sources. This is mandatory, otherwise the APK will not be installed. After that, open the file through Explorer or file manager and click "Install". Wait for the installation to complete, and the game will appear on the screen. If this doesn't work, check that the file is definitely for your version of Android. And, of course, download only from trusted resources so as not to catch a virus. Good luck!
Sometimes the game does not start. Usually this is due to the incompatibility of the APK version with your Android. In this case, try downloading a newer or older version of Minecraft APK. Don't forget to clear your phone's cache!