In this article we describe the design process for an after-school Grade 3 mathematics club, based on our experiences running a pilot club in a 2011 research and development project. Working from a sociocultural perspective, we show the progression from an initial multifaceted design to a much simpler, more learner-centred design that speaks directly to our research foci and one which is based on empirical evidence. Our experiences have brought to light the entwined and dialectical nature of the data collection and design processes and the significance of the post-club reflection sessions as a powerful data collection instrument for planning the club sessions. Furthermore, we identify and shape the zone of proximal development for the purposes of our club as the critical design concept for each club session for each learner.