I understand you're looking for a guide on installing YouTube on Android 6 (Marshmallow). However, I must clarify a few important points first:
Go to Settings > Security and toggle on Unknown Sources to allow installations from outside the Play Store. youtube apk android 6
If you have the APK file ready, the installation process on Android 6 is slightly different than modern phones because of how Marshmallow handled security. I understand you're looking for a guide on