This can be done at any point in the game, in any sport, and in free roam or an event. you must survive a crash landing between 90 and 99.9Gs. Anything above 99.9Gs is a KO. Specifically, as shown in the video, head to Pointe De Mandallaz in the Aravis area (the starting area). If you have not been to this point on the map, the coordinates are 4523S - 2629E. Head to the dilapidated house in this area and use your wing suit to attempt to bounce off the rock just below the hill. You can retry this line over and over again.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUKDDGWFoBmJ...