It is so easy and yet so fearful the first time

Get everything together, slapper loaded with ink, tasty stuff for pigs, apples, grapes, sweet potatoes etc.
Then, as pigs are gorging, give them a hardish slap on the bum, once on each side. Imagine you are slapping someone who you find really annoying, but not someone you would like to SLAP HARD
You should do this as the pigs are being loaded or the evening before. Good luck

and remember, practise makes perfect