How do you recruit the mages instead of the templars on Dragon Age Origins

Firstly you need to have made the right dialogue choices during the way, like recruiting Wynn.

Secondly, you need to save the mages during the final fight at the top of the tower. Use the Litany as instructed to stop the demon turning them into abominations. When the fight is over, speak to High Enchanter Irving, who will give you the Circle mage's support.