Fastest way to turn screen off / lockscreen

(*)Uninstall apps
Go to your device's Settings app .
Select Apps or Application manager.
Select App you want to uninstall.

Touch Manage Device Administrators > Uncheck Screen Off > Deactivate > Uninstall.

The system requires "Android.permission.BIND_DEVICE_ADMIN" permission to perform a quick turn off the device. Please grant this permission for the application to function.