Use case driven analysis is the corner stone of software and systems modeling in UML and SysML, respectively. Although many books and tutorials have discussed the use of use case diagrams, students and industry practitioners regularly face methodological problems in writing good use cases. This paper defines a methodological assistant that helps designing use case diagrams relying on formalized rules and reuse of previous diagrams. The methodological assistant is implemented in Python. It is interfaced with the free SysML software TTool, and with Cameo Systems Modeler.