Background: During initial teacher education, reflective thinking can have a key role to play in preparing pre-service teachers for professional practice. Therefore, the approaches taken to assist reflection are important to optimise learning from classroom experiences. Purpose: This case study from Finland sought to examine the development of pre-service teachers' reflective thinking during a teaching practicum in which a non-prescriptive, self-guided approach to reflection was implemented. Method: Drawing on best practice, we designed an approach to reflection which placed emphasis on the agency of the pre-service teacher, and involved elements including dialogue, peer interaction and collaboration, and video. The research followed three pre-service teachers who worked in a peer group with their mentor-teacher during a six-week teaching practicum where this approach was employed. The peer-group mentoring discussions were recorded and analysed qualitatively. Findings: The in-depth analysis of data identified three different sequences of self-guided reflection phases, with findings revealing how the pre-service teachers' reflective thinking developed according to their individual and shared trajectories. It suggests that for practicum experiences to be reflected on in a way that supports deep learning, the process must stem from pre-service teachers' individualised learning needs. Conclusions: This case study highlights the rich potential of a non-prescriptive, self-guided approach to reflection as a tool for use during initial teacher education. It can offer a more personalised and active way for pre-service teachers to learn from early teaching experiences, gain practical understanding and develop their reflective thinking.