Research is the most fundamental aspect of how to make a documentary film and is the cornerstone of documentary pre-production. This research process does not just mean reading articles about the subject, but instead trying to get as deeply involved as you possibly can. Research is important for narrative film projects as well, but what is different is that you can actually start the production on your documentary film while you are still researching.
Part of your documentary pre-production process is going to be making contacts and estimates about what kind of access and materials you will practically be able to get. It is difficult to draw out the themes of turbulent family life and sexual issues in Crumb if Robert Crumb and his family are not willing to be open, so you are going to need to know this. This is not so cut and dry as an answer from them in an email, and you may actually need to spend quite a bit of time with them before you will know. Try bringing your camera during this pre-production contact period even if you are not going to be filming.
You are also going to need to look at the availability of stock footage, news footage, outside footage, stills, pictures, music, and other elements that are not created by you but are essential to the topic. This actually makes up a large amount of the media material in most documentary films, which is why the question of how to make a documentary film is really close to how you are able to arrange already existing work.