There is not much described what they did to her, but there was definitelly much beating, something with her fingers and especially with her nails. And I think when sorceress looses all her power, it's also very unpleasant (if you don't reed book there is part, where is described how much this affected Ciri, when she 'refused' all her power).
Her hands .. There are mentions about her hands, all covered with her blood (
they used some torturing tool, which they put on her hands and fingers).
Also - not exactly torturing - but some fight with Bonehart, but I am not sure who actually was beaten more, she (even with her wounds) was brave and defended herself quite well