Hi,
Slapmarks should be on each shoulder and done when standing behind and to the side of the pig.
Just a firm slap is needed - no need to try and knock the pig across the room - you are trying to just penetrate the skin, not drive the tattoo pins fully home!
Do it as far in advance as you like, or even as you load into the trailer - we aim for a couple of weeks before slaughter if possible, although it doesnt really matter.
Thanks