Many innovative teaching approaches lack classroom impact because teachers consider the proposals impractical. Making a teaching approach practical requires instrumentality (procedures), congruence (local fit), and affordable cost (limited time and resources).This paper concerns a study on the development and effects of a participatory design based teacher training trajectory aimed at making guided discovery learning (GDL) practical for student biology teachers. First, we identified practical heuristics for designing GDL lessons by analyzing design protocols made by biology teachers who are experts in GDL. Next we inventoried student responses to their regular lessons and to GDL based lessons. Based on this we prepared a teacher training program for eleven student biology teachers in which they applied the heuristics and stepwise extended their teaching repertoire in the direction of GDL. The participants' design processes and resulting lesson plans were scored on both use of design heuristics and GDL characteristics. The participants were interviewed about their motivational beliefs before and after the program. Results showed that student teachers are able to design GDL lessons and used the heuristics to design GDL lessons. Their motivation for implementing GDL in their classroom had increased substantially. The paper concludes with a critical reflection on our method of participatory design and its applicability.