Start off with is folded up so it appears just as a square, get your gorse used to this then unfold it slightly so it is slightly bigger and makes a bit of nose, if your horse doesn't like this go back to having it folded and rub it against your horses body then slowly unfold it once again repeat this until your horse isn't fussed about it being unfolded then unfold it more and repeat and so on until its completely unfolded and again rub it against your horses body to show that it is no harm. Then take it in your hand and scrunch it and play with it so that it makes noise then show it to your horse, take it slow eventually your horse should get used to it, once your hose is fine with having the plastic bag close to it, show it the bag (not flapping) and then let it flap slightly repeat this at different distances away from your horse and then let it flap some more after it is fine with the flapping tie the bag to a fence post and walk your horse past it until your horse no longer cares and then increase the number of bags and introduce then in different sizes and colour and just lead your horses past them.
I know this is a long explanation but I hope it helps.
Horsemanship on the round pen then introduce the plasic bag and let the horse look at it and sniff it. Then try to touch them with it and reward when they let you. If the horse spooks just try again and take it slow.
Put a plastic bag on a stick or something and rub it on them until they stand still, then once they stand still, rub it on them until they stand still. (You may want to be in a round pen or small area and depending on the horse, you may or may not want to have a lead line on them