It is very hard to use the follow me drone skiing functionality, not least due to the speed of the skier, but the fact that the drone maintains the same altitude whilst the skier descends, so at best you'll only manage 10 secs of decent skiing.
Because this feature needs a strong GPS signal and use of the drone's vision systems, it's only available when you enable P-mode (Positioning mode). Make sure your drone is set to P-mode so you can use ActiveTrack. Once you're in ActiveTrack mode, go ahead and tap or draw a box around the subject onscreen while using DJI GO 4 app or DJI xIrH398.