This paper presents an overview of a generic task model of music composition, developed as part of a research project investigating methods of improving user-interface designs for music software (in particular focusing on sound synthesis tools). The task model has been produced by applying recently developed task analysis techniques to the complex and creative task of music composition, avoiding any attempt to include the aesthetic motivations and concerns of composers. The author then illustrates the application of the task model to software design by describing various parts of Modalyser, a graphical user-interface program he designed for creating musical sounds with IRCAM's Modalys physical modelling synthesis software.