Methodologies are one of the most significant key factors to the success of project development. Since there is no single methodology that can be uniquely pointed as "the best", the discipline of situational method engineering (SME) promotes the idea of creating method components, rather than complete methodologies, and tailoring them to specific situations at hand. In this paper we present a holistic approach, called ADOM-SME, for representing method components and tailoring them into situational methodologies. This approach, whose roots are in the area of domain engineering (also known as product line engineering), supports specifying the five main methodological aspects (products, work units, stages, producers, and model units), as well as instantiating them into endeavour concepts, using a single frame of reference. Furthermore, the proposed approach enriches the standard metamodel for development methodologies, ISO/IEC 24744, by supporting the creation of valid situational methodologies and guiding their tailoring.