As projects for developing information systems are getting larger and more complicated, we need to have more advanced development methods suitable for every development situation. Method engineering is the discipline to construct new methods from parts of existing methods, called method fragments. To achieve this objective, we need to clarify how to model the existing methods and how to assemble method fragments into new project-specific methods, so-called situational methods. Especially, to produce meaningful methods, we should impose some constraints or rules on method assembly processes. In this paper, we propose a framework for hierarchical method modelling (meta-modelling) from three orthogonal dimensions: perspectives, abstraction and granularity. According to each dimension, methods and/or method fragments are hierarchically modelled and classified. Furthermore, we present a method assembly mechanism and its formalization as a set of rules. These rules are presented in first order predicate logic and play an important role in the assembly process of meaningful methods from existing method fragments. The benefit of our technique is illustrated by an example of method assembly, namely the integration of the Object Model and Harel's Statechart into Objectcharts.