In Magento 1, logging was pretty simple, we add Mage:log($var, null, 'test.log'); anywhere you needed to know something.

With dependancy injection in Magento 2, things are a little different.

There are two main ways to log things.

Injecting \Psr\Log\LoggerInterface or using the \Zend\Log\Writer class.

Normally, if the logger is already injected, I’ll just hijack it and use it:

$this->logger->debug($var);

If not, just declare it and use it:

\Magento\Framework\App\ObjectManager::getInstance()
    ->get('Psr\Log\LoggerInterface')->debug('message');

But what if you are outside the scope of \Psr\Log\? Use the simple, inject \Zend\Log\ like below:

$writer = new \Zend\Log\Writer\Stream(BP . '/var/log/test.log');
$logger = new \Zend\Log\Logger();
$logger->addWriter($writer);
$logger->info('Your text message');

How do you know if the logger is already injected?

Look for the $logger variable in the class you are attempting to log from, and in the constructor.

 /**
  * @var \Psr\Log\LoggerInterface
  */
 protected $_logger;


 public function __construct(
         \Magento\Framework\Model\ResourceModel\Db\Context $context,
         \Magento\Store\Model\StoreManagerInterface $storeManager,
         \Magento\Eav\Model\Config $eavConfig,
         Product $productResource,
         \Magento\Catalog\Model\Category $catalogCategory,
         \Psr\Log\LoggerInterface $logger,
         $connectionName = null
     ) {
         $this->_storeManager = $storeManager;
         $this->_eavConfig = $eavConfig;
         $this->productResource = $productResource;
         $this->_catalogCategory = $catalogCategory;
         $this->_logger = $logger;
         parent::__construct($context, $connectionName);
     }

If this is the case, then just hijack it and use:

$this->logger->debug($var);