One of the most problematic issues in healthcare is the patient's lack of adherence to the therapy. Patients' motivation is indeed hard to maintain when they have to execute repetitive, boring or tedious exercises. In such cases, they tend to practice less regularly and even to entirely give up the therapeutic protocol. Fortunately, therapeutic exercises can very often be turned into compelling games. Such \textit{therapeutic games} are considered as a very promising solution to the patient adherence problem. Yet, therapeutic games are very complex to design : 1. the gameplay is particularly constrained, e.g. the game has to both motivate the patient and provide the therapeutic effects 2. the game must be evaluated on both its medical and motivational results, and 3. relevant health knowledge is hard to share between health experts and game designers. In this paper, we propose a game design method for therapeutic games that provides guidance for every step of the design, along with tools for every design challenges we identified.