After a study, we established that one of the obstacles of the adoption of Serious Games (SGs) by teachers was that they cannot shape their educational scenarios to their specific teaching context. The work we present in this paper tackles the general problem of designing tools to help them customize the educational scenarios of SGs. Our approach is to provide a model suited to describe SGs that are composed of several stages, and to provide its implementation in an authoring tool in order to help the teachers to visualize, modify and check the consistency of the scenarios. The evaluation of our model shows that it is capable of describing most of the SGs we targeted, and the first user tests of our authoring tool prototype are also promising.