Early on in the Blood & Wine expansion, Geralt will get to Beauclair and be welcomed by a child who carries a letter for him. The letter triggers a side quest, "Turn and Face the Strange", which takes you to a laboratory where the mutation ability becomes available. The game will show you a quick tutorial, but basically you will require greater mutagens of various colors (Red/Blue/Green) and 2 ability points to start a mutation under a new tree found in the Character menu. The achievement unlocks once the mutation has been developed.