This paper describes how an interdisciplinary design team used the Four-Component Instructional Design (4C/ID) model and its accompanying Ten Steps design approach to systematically design a professional development program for teaching differentiation skills to primary school teachers. This description illustrates how insights from a cognitive task analysis into classroom differentiation skills were combined with literature-based instructional design principles to arrive at the training blueprint for workplace-based learning. It demonstrates the decision-making processes involved in the systematic design of each of the four components: learning tasks, supportive information, procedural information, and part-task practice. While the design process was time and resource-intensive, it resulted in a detailed blueprint of a five-month professional development program that strategically combines learning activities to stimulate learning processes that are essential for developing the complex skill "providing differentiated instruction in a mathematics lesson."