We see Julia at the very end of the book, and can get a picture of her physical appearance, and she mentions a couple things from which we can infer what happened to her. We don't know exactly, in detail, but we can get the general gist. When Winston meets her, the novel writes that
"her face was sallower, and there was a long scar, partly hidden by the hair, across her forehead and temple...her waist had grown thicker, and, in a surprising way, had stiffened...her body felt like [a corpse]."
So, instead of being young, flexible, friendly and full of life, she is pale and stiff, like all of the life had been sapped out of her. She has a scar, so, her torture involved some sort of awful facial cutting or damage.
To get an idea of what they did to her, we do know that they did something to make her betray Winston, just like Winston did her. It's the first thing she says to him. She said that they threatened something so horrible that in order to escape it, she told them to do it to Winston instead. That is almost exactly what happened with Winston. Other changes are startling--she is filled with distaste for Winston, doesn't really want him to follow her, and after a bit when he first sees her, tries to get away, but then gives up. She used to seek out their meetings, and spend her life evading the Party to meet him. Now, she distastes anything having to do with her former life, and is very passive and gives up easily. They have taken her spirit and zest, and left a hardened shell behind.
So, her torture ended in some that was very similar to Winston's, was physically brutal, and completely changed her personality and nature, breaking her will and taking the life out of her. I hope that those thoughts helped; good luck!