For pigs, a few days before I need to load I park the trailer in the corner of their patch and arrange the electric fence so they can go up the ramp but not round the sides. put their feed inside, they soon learn to go in.
The problem I have is getting them out at the abattoir. Had a problem with goats and sheep last week. I'm sure the men have a good laugh each time I go watching me trying to push animals out.. Pigs have nothing to get hold of.