Spatial planning as a field in continuous transition needs a way of working that allows for questioning its professional practice. In this paper, we focus on the design of 4-days long 'studios' for planning professionals that aim to reflect on new notions of democratic practice and participatory planning. During these studios, different methods to enhance a collective reflective attitude among participants were tested. The paper describes how the tutors of the studios tried to encourage participants to develop their own theory of practice by iteratively (re)designing the learning artefacts, learning content and learning modes of the studio. In the conclusions of this paper, we introduce the idea of a Participation Studio Conjecture Map to support and structure a culture of collective reflection-in (participatory planning) organizations.