All three of these stages take place in Part III. Having completed the first phase of his treatment, “learning,” Winston moves to the second stage, “understanding,” which he must complete before being allowed to advance to the third stage, “acceptance.”
The Party seeks obedience above all else, which includes loving Big Brother.
Not one soul in the Party will die a martyr against Big Brother, this is the purpose of the Ministry of Love. Even more than that, every member of society will be made to love Big Brother, then he will be shot in the back of the head before he or she can change their minds. Winston learns in the first part of Part III. He learns that he will never know if the Brotherhood exists, he learns that Big Brother is forever. He learns that two plus two is five. His next stage is acceptance in which he must accept that the Party is forever and cannot be overthrown, therefore he must find a way to be a part of it. He must accept that the Party seeks power above all else, absolute power, which is why they spend all this time with dissidents. They want power over everyone down to their thoughts and they want to wield their power to make everyone love Big Brother through suffering so that they know beyond a shadow of a doubt that they possess the loyalty of minds. In Room 101 Winston faces his final stage of understanding through enduring his worst fear- the rats. It is here that he betrays Julia, that he wishes she could be in his place enduring the rats and it is here that he learns to finally understand and love Big Brother.
In chapter 3 of part 3, O'Brien tells Winston of the three stages of Winston's reintegration: "There is learning, there is understanding, and there is acceptance." He means that first, the Party will drill their "truths" into a prisoner's head through torture, then by ending the torture (it is here that Winston comes to love O'Brien because he stops the torture), the prisoner will come to understand that Big Brother and the Party will take care of him, and finally, the acceptance is when the prisoner actually and completely believes everything that Big Brother and the Party tells him. Above all else the Party, according to O'Brien, is more interested in the thought than they are in the deed. They are after control of thoughts because they know that if they can control people's thoughts, then they can easily control the "overt act".
Learning is when he is forced to accept that 2+2=5 under torture. Understanding is when Winston understands that the torture will be only inflicted on him until he passes the burden to someone else, and that he has to accept that Big Brother is eternal, that 2+2=5. Acceptance is when he becomes fully dedicated and loyal to the Party and its cause, where he awaits execution willingly to prove his devotion to the Party.