Teacher professional learning has been recognised as a critical contributor to improving teaching quality and securing student learning. Leadership at different levels is important for professional learning in schools. Job-embedded teacher professional learning occurs in collaborative activities that include co-teaching, action research, and initiation and implementation of curricular and instructional change. The current article conceptualises a non-linear, iterative, and recursive process of teachers leading initiatives collaboratively in schools. This conceptualisation was based on an analysis of 38 interviews with teachers and school leaders, field notes from 50 visits to four Singapore primary schools, and email exchanges. The process has three stages -- "sharing," "improving," and "spreading." In this process, "affective support" is central in providing social reinforcement to keep the team motivated to revise and move a change initiative forwards. The article enhances understanding of teacher collaboration process for change in schools, through which teacher leadership for professional learning is evidenced.