The second half of 1984 is all about torture. Winston is taken to the Ministry of Love, which is a machinery of torture: it has torture chambers within torture chambers.
First, Winston is broken physically. He is beaten and starved. Some of his teeth and hair fall out.
Then, he is psychologically tortured by O'Brien, who may very well be Big Brother himself. O'Brien calmly begins the process of turning Winston into a unperson. He uses his mother and Julia against him: women are all traitors; one must love Big Brother only.
To make sure his psychological re-conditioning worked, O'Brien gets Winston to unthink even his most basic thoughts by admitting that 2 + 2 = 5.
Then, O'Brien uses Winston's worst fear against him: rats. He puts Winston's head in a cage full of rats until Winston confesses to being guilty of thoughtcrime.
Finally, Winston is released to meet other reformed thought criminals, including Julia. We never know if she was a thought criminal or a spy for Big Brother. But, Winston treats her as he should, by calling her "sister" instead of lover, which is the emotional equivalent of admitting that 2 + 2 = 5.
Are you asking about what happens between this and the end of the whole book? If so, a very brief synopsis is all I can give.
They wake up and very soon afterward they are arrested by the Thought Police.
After that, most of the story is about O'Brien trying to get Winston Smith to see that 2 + 2 = 5. In other words, O'Brien is trying to get Winston to become a regular member of the Party, believing everything Big Brother says. He does this mainly by torture.
Eventually, Winston is released, but he doesn't quite believe. At the end of the book, though, he finally believes and he finally loves Big Brother. Now, he says, he can die...