You interview users as they do real work in their real work environment. Then you and your team analyze the data.
First, you must prepare by gathering your team and selecting the participants. [Details]
Next, you conduct approximately day long "interviews," watching what the participants do, and asking them about it. [Details]
Finally, you analyze the data you collected, creating a bunch of representations of the work in all its similarity and variety. [Details]
On a closing note, all this great data collection and analysis will be for naught if the information does not get into the hands and heads of those who can put it to good use.