Generalizing is a key element in mathematical reasoning as well in early algebra and algebraic thinking (Kaput, 2008). However, students find it difficult to understand the meaning of generalized arguments, which means that teachers need to support students to develop their generalized argument (Stylianides & Stylianides, 2017). A way to increase teachers’ possibilities to support students’ reasoning may be through intervention studies. Stylianides and Stylianides (2013) write about the importance of research-based interventions of proof and proving in teaching. Since generalized arguments can be seen as a part of proof and proving (Stylianides & Silver, 2009), intervention studies also about generalized arguments should be important. Similar reflections have emerged in the Argumentation and proof group at CERME where a lack of design-based studies promoting investigation in the classroom and a need to shift research focus from the learners to the teacher is emphasized (Mariotti, Durand-Guerrier, & Stylianides, 2018). This poster will exemplify a part of a cyclically recurring intervention process by answering the question: What challenges do teachers meet when trying to understand a given Design Principle (DP) (McKenney & Reeves, 2012) and design and implement teaching based on it?