2 things to keep in mind is to not lie and also ideally not talk negatively about the company. As much as you had a bad experience, it might not reflect well on you if you start bringing up all the bad things about the company. (since there's no way for the employer to be sure who's side of the story is true) If you have to mention more detail, try to explain in a way that's objective and factually. Raise points that have affected you without bringing personal feelings or emotions in.