One young Mom I know thinks it begins with sleep training.... (she'd recommend the book Healthy Sleep Habits happy Child by Marc Weissbluth)
Another young dad I know believes it starts in the high chair (around 8 or 9 months) the first time a child spits food at you or throws their food intentionally on the floor (not just a normal spill or messiness.) The parent can lovingly but firmly say in a sing songy way, "Oh no... We don't throw (or spit) our food... all done." Then simply set the child down. If you are worried about them going hungry you can pop them back in the high chair a half hour later to finish. What if they cry? Hold them, sooth them,.love them (provide empathy) but they do not get to go back in the chair right away. After awhile they know "Oh no" is not good. A neural connection has formed in their brain.
Then there's a grandmother I know who taught me that with her (now grown) children it began with the "wiggle worm on the diaper table." If they would not hold still for diapering she would simply hold then still gently with her arms and body. She would release them after a minute and try again. They did not like this restraint. She would gently say, "Hold still" while she did this" She'd repeat this as often as she needed to and soon they learned what "hold still" meant.