Add this to wp-config.php

define('WP_DEBUG', true);    // Enable WP_DEBUG mode
define( 'WP_DEBUG_DISPLAY', false );  // Hide errors from browser. Set to true if you want to see errors in browsers as well.
define( 'WP_DEBUG_LOG', true );  // Enable Debug logging to the /wp-content/debug.log file

The log file will be available at /wp-content/debug.log