The aim of our approach is to improve the requirement specification step, which is very important during system development. But rather than study languages, models and tools in the requirement specification context, we work on the formalization of the design specification we call method. Our goal is to describe a multi-method system, a system which allows the user to define his own method or to use predefined ones. During the design specification step, the system will guide the specifier in terms of the chosen method.