Here are some primary notes: You did not mention not the field of your research or the opted title of your research. This will help researchers to advice you significantly. It is good that you mention that you deal with a qualitative research. As a doctorate research, it should be based on several theories in order to apply a comparison study and to get a thorough and comprehensive findings. Further, you should investigate your hypothesis and questions based on the opted theories in separate chapters. In the introduction you should specify your research questions and goals, your methodology and a comprehensive literature review.

Dear Andreja,

you probably have to decide first if you are more interested in testing theories or in the cases you want to investigate in. This determines much of case selection and case study methodology. You might start with Harry Eckstein (1975) or Arend Lijphart (1975), who wrote some basic articles on case study methodology. You could also look at John Gerring's work on case studies (2004).