How to start Developer Mode in Magento 2

To enable Developer Mode in Magento 2, use SetEnv MAGE_MODE “developer” in your .htaccess file. Please note that this method works only in case when AllowOverride All is situated in a proper <Directory> directive. And don’t forget to check if the Apache process user has write permissions to all necessary directories.

There is also another way to start Developer Mode in Magento 2. I don’t recommend you to use it, as it is a dirty solution, but you can simply put  $_SERVER[‘MAGE_MODE’] = ‘developer’; at the beginning of your index.php file to run the mode.

More tips from Magento 2 Developer’s Cookbook

Source (Magento Stackexchange Question) (Bryant Luk)