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. The model itself describes the purely practical aspects of music composition, avoiding any attempt to include the aesthetic motivations and concerns of composers. We go on to illustrate the application of the task model to software design by describing various parts of Modalyser, a graphical user-interface program designed by the author for creating musical sounds with IRCAM's Modalys physical modelling synthesis software. The task model is not yet complete at all levels and requires further refinement, but is deemed to be sufficiently comprehensive to merit presentation here. Although developed for assisting in software design, the task model may be of wider interest to those concerned with the education of music composition and research into music composition generally. This paper has been developed from a short presentation given at the First Sonic Arts Network Conference in January 1998.