We present results from a course, "Informal Science Education for Scientists: A Practicum," co-taught to graduate students in STEM-related fields by a scientist/engineer and a social scientist/humanist. This course provides a structured framework and experiential learning about informal science education during a semester-long experience. The data collected across six years of the course (11 less than or equal to n less than or equal to 16 for each) provide strong evidence that the course has been effective in encouraging graduate students in STEM-related fields to feel more skilled at and confident with informal science education. Details are provided as to how manipulation of the course structure (i.e. making it project-based, emphasizing understanding audiences, stressing the iterative nature of design, and increasing evaluation research training) influenced the student outcomes.