Encourage a palm facing up follow through. Lead the throw with the elbow. Follow through by pointing throwing hand at the target. Don't worry about it with beginners. Just more practice is required to get those those finger muscles strengthen and the flick automatic.
Most beginners try to throw the disc rather than flick it.
Once the basics are there, the distance will follow. Check that the grip is not finger tips only and the swing is not circular, but in line to the target. Return the basics. To remind yourself what it was like to learn, try throwing opposite handed for a while. We are still a young sport and good coaching techniques are yet to be discovered.