I wouldn't class marking as training, as they usually pick it up themselves,
The easiest way, is to take a young dog ferreting, watching the rabbits hitting the nets, and using there nose, eventually the penny should drop, and they could start marking.
All dogs are different, and its up to the owner to notice, I have seen markers digging at a bury and whining, this you don't want, it alerts the Rabbits that you are there.
The best ones i have seen, has just been a tilt of the head, nothing more.
The best marker i ever had, was my Whippet X Bedlington, just a tilt of the head, then she