The most straightforward approach is to pick your target group and do formal interviews. Formal interviews follow an interview guide and are administered with the respondent fully understanding they are being interviewed. Informal interviews also provide qualitative data, but follow the flow of a natural conversation. Data from informal interviews can provide unique insight, but may be difficult to analyze. You can take notes or record your interviews. Open-ended questions provide the most qualitative data.
Watch and take note of the nuances of your case including the atmosphere, the interaction between different types of people, social customs, and even your own feelings during the research. Participant observation is a popular form of directed observation where researchers seek out the place they want to research and actually get involved there, such as Douglas Harper did when he became a farmer to learn about rural America.
Analyzing documents is considered qualitative research. This can come from concept or theory synthesis in a literature review or from primary sources gathered during field work. Some people can use documents to perform quantitative research, as in the comparison of figures or numbers within studies. But, this is not how many people conduct "desk research." Documents can provide depth to your case study and fill in gaps not fulfilled by your field work.
Action research is inherently qualitative because it requires intensive interaction and reporting. This approach is less systematic and is highly specific to the place, issues, and participants. Action research involves cycles of reflection and planning that lead to changes within the group and their larger community. Researchers can coordinate it around a project, such as making a video or creating an organization. The researcher serves as facilitator or only as an observer, documenting the process.