User stories, use cases that fulfill business requirements are the basis of functional reqruirements. Personas and stakeholders have to be kept in mind. To acquire this information I would start with standardized questionnaires. These questions are slightly modified for certain projects, and of course for questions I already have answers to. Once I have this information, and have started off documentation, I engage in email, telephonic, skype, conversations. Get the information required, and create the FRS. This may also come with wire frame if required.