From what I have seen, cum training of this type is basically a form of classic conditioning. You switch the physical stimulus for another one, i.e. you swap the food (the clitoral stimulus) for a bell (the command) and the dog salivates (the lady cums).

However, humans, unlike dogs, have a little more resilience to this sort of thing so as stated it will take time. Conscious control is stronger (though this is largely offset by the subs consent) and there are other complications in the human psyche

If your Master wants to achieve this sort of thing he needs to start by associating the command with both the physical act and the pleasure - i.e. he needs to do both at once. Then you remove the physical in slow stages (less and less stimulation) and if at any point you lose the orgasm go back to the previous step.