The three stages of Winston's integration can be described as, first, outward capitulation to power; second, inward assimilation of the will of the Party; and finally, betrayal of the one he loves so that he has no independent loyalty apart from the Party.
Physical torture, or what O'Brien calls the boot stamping the face, leads Winston to outward obedience and capitulation to the regime. This is the easiest step in his reassimilation, and it begins from the time of his arrest, when a rubber truncheon to the elbow brings him to his knees. Under torture by O'Brien, Winston confesses and physically obeys the Party.
But that's hardly enough for the Party: they want him to really believe what they tell him, and they also want to own his soul. The second part of his reintegration, which also involves torture, convinces him to accept that two plus two equals five. He can't simply say it, he has to believe it to be true. He has to utterly assimilate the idea that there is no objective truth independent of the Party's will.
Even that, however, doesn't satisfy the demands of the regime. Winston still has a sense of integrity apart from the Party because he has never once betrayed Julia. When O'Brien takes Winston to Room 101 and puts the cage with the huge, starving rat over his head, Winston, in terror, betrays Julia, asking with complete sincerity that the rat be used to attack Julia instead. At that point, he is broken, meaning he can be released back into society.
As mentioned by the previous educator, there are three stages of Winston's reintegration: learning, understanding and acceptance.
Each of these three stages comes with a very specific form of torture, designed to break Winston down and make him realise that resistance against Big Brother is futile.
In the first stage, for example, Winston learns that the party is all-powerful and will use any method to bring about his domination. We see this through the violent beatings that he suffers and the "Party intellectuals" who inject Winston with an unknown chemical. O'Brien also encourages Winston's learning by saying that Julia has betrayed him; a fact designed to cause distress and demonstrate that the party can break anyone down, even the most rebellious.
In the second stage, O'Brien increases the intensity of Winston's torture with the introduction of a machine called the dials. This machine enables O'Brien to inflict pain on Winston (controlled by a lever) as he questions him on the party. If Winston gets a question wrong (does not answer according to party ideology) then O'Brien cranks the lever, causing more pain:
O'Brien had pushed the lever of the dial up to thirty-five.
"That was stupid, Winston, stupid!" he said. "You should know better than to say a thing like that."
Finally, the third stage of Winston's reintegration is represented by Room 101. There is much mystery surrounding the contents of this room. When asked, O'Brien tells Winston that it contains "the worst thing in the world." When Winston is sent there, in Part Three, Chapter Five, he is confronted by rats, his ultimate fear. He is so overcome with terror that he almost immediately relents:
Do it to Julia! Do it to Julia! Not me!
O'Brien has thus demonstrated the effectiveness of the three stages of reintegration. Having betrayed Julia, the woman he loved, Winston is no longer considered to pose a threat to the party's supremacy. He is released from the Ministry of Love, completely reformed.
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.”