Think about how best to approach people in the group or culture you want to study, based on your preliminary stage proposal. Do a switch-places exercise and assess how you would feel if someone asked to put you under a microscope.
Determine whether you will need a translator. Even though English or French may be the official language in some countries in West Africa, for instance, it does not mean rural people can speak either one. The same applies to "official" languages in other regions of the world where translation may be essential to your work.
Find out whether another researcher has worked in the area. If so, ask for advice. If the person has credibility with the people of the group you want to work with, she may be able to introduce you.
Identify the key person to approach in the group you want to study. Start at the top and work your way down. If you have already met a person in the group, he could "take you to their leader."
Ask politely. Outline your reasons for wanting to study this particular group of people. Make clear what you will bring to them and what you want to take away.
Prepare for the worst while hoping for the best. Don't assume the group you want to study will welcome you with open arms. Use a friendly, cautious, but confident approach, indicating that you realize they are doing you a favor.
Leave something of value. When you have done your research, thank the people who cooperated with you. You may even want to get involved in helping to resolve the problems you found in your research.