Let your horse sniff the ball. You can gently rub the ball all over your horse's body to show him/her that it is harmless.
Begin with approaching retreat.
Just spend plenty of time showing it to them and letting them sniff it, if you've an arena or area to turn them out with it and let them investigate it that'd be great! Once they're not afraid of it while it's still you can work on moving and bouncing it around!