Rad Steez, Bro! is an achievement/trophy you can acquire in the Hearts of Stone expansion for the Witcher 3: Wild Hunt. To unlock this trophy, you’ll have to slide for 10 seconds uninterrupted. You can use many of the mountains that dot the Skellige islands, but the easiest way is from Yngvar’s Fang on the north of An Skellig.
Take a second to catch your breath, then walk down the slope until Geralt starts sliding down the mountain. Slide all the way down to a small rocky area in front of the entrance to a cave.