The use of serious games and gamified softwares is a new and growing trend for training professionals in a wide variety of disciplines where procedures and decision-making are key (automotive diagnostic, surgery, etc). Serious Games are safer, less expensive and advocated to be more efficient. Unfortunately, there is a lack of methodology and tools adapted for non-computing experts to develop their own gamified learning scenarios. In this paper, we propose an approach allowing trainers to model professional activities in the form of serious games scenarios. Trainers are enabled to express their expertise using a domain specific graphical representation which will be implemented eventually in an easy-to-use authoring tool. The produced scenarios describe both the actions necessary for completing the professional procedure and the associated pedagogy to provide the learner with relevant educational feedback. The proposed approach specifies a model matching those requirements, and is illustrated by an application example in the automotive context. We intend to demonstrate that an appropriate model is likely to make scenario editing accessible to trainers who are not necessarily familiar with computer modeling in the first place.