In addition to the points Hitesh mentions above, qualitative research is really not "quantifiable" but instead helps understand direction, fine tune hypothesis, explore a number of options (samples, concepts, ideas) which would not be feasible doing at that stage through quantitative research.
Key for success is the experience and quality of the moderator. without that, both the inputs during the research and the report (which requires interpretation skills and ability to use implicit /projective techniques) could be misleading.
Often, big global agencies don't really have seasoned and skilled moderators in every country, and its best to leverage local freelance.
there are several books written on the topic and literature available as well.
1. Define your objective
2. Define and identify your target audience
3. Identity right tool for your audience to engage ( Paper form, online link, phone calls etc )
4. Make you're objective simple and easy to understand to your audience
5. Try to get specific answers. Or quantifiable answers.
6. Prepare a proper system to compile the results and to generate results.
Which is the goal of this research? Identify customers needs, improve the products and set up effective marketing actions.
Ask open questions inviting people to revert with detailed replies, not just a yes or no. Indeed you need to identify the right targets (but not too many persons), ask relevant questions and schedule the proper time (this search can't be too short) to be successful. Quite a long time is then needed for the final report to highlight and summarize your findings.