This unique quest will allow you to control Ciri for few minutes. It starts automatically during the Bloody Baron main quest. More specifically, it will start after the meeting with the baron in Crow's Perch (M3,3).
You start by walking alongside the river. Ciri will find a group of wolves. You must defeat them only with a blade - you can't help yourself with signs or other inventory items. It is best to attack wolves one after another, more frequently than usual. Don't forget about dodges and jumping away as well. After meeting Gretka, follow the girl. Deal with another group of wolves and reach the place where Ciri will find corpse. You must examine it thoroughly by picking all options from the list. It will allow the girl to deduce that she is dealing with a werewolf and that she must prepare a specific oil for her weapon.
You must collect four ingredients - 1 dog tallow, 1 wolf's liver, 2 fool's parsley and 3 wolfsbanes. Obtaining all ingredients is pretty easy. Dog tallow and wolf's liver can be found by examining the bodies of defeated wolves. Fool's parsley and wolfsbane can be found by searching for white and blue interactive plants. Collecting all items shouldn't take you long. After you're done with it, Ciri will automatically prepare the oil and put it on her weapon.
Follow Gretka to the cave where you will find the King of Wolves. Although the beast has longer health bar than regular monsters, defeating him won't be hard (thanks to the oil). Remember about dodging and jumping away whenever Ciri will loses more health. This will allow her to regenerate. After a victorious battle, listen to the conversation with the peasant. Soon you will regain control of Geralt and will be able to finish the Bloody Baron quest.