The demand for teaching and learning collaborative design is probably greater than ever thanks to many influential parties highlighting creativity, collaboration and designing as necessary future skills. However, 'good' collaborative design necessitates a clear understanding of what constitutes 'good' and 'collaboration', and of how an individual's activities contribute to a team achieving a satisfactory design solution. This research aimed to develop a methodological approach to facilitate the analysis of collaborative interaction and design as equally meaningful and closely linked components, and to equally account for verbal and sketching activities. The developed extended linkography accounts for verbal and sketched design moves and utilises mobile gaze-tracker data to confirm visual attention and linkages between sketches. The method has potential importance for research on design cognition and collaboration, but also for analysing collaborative problem solving in other contexts. To illustrate the method, a case study of packaging design students collaboratively designing a sustainable Christmas basket is presented. The gaze data revealed that students' collaborative engagement with 'shared' sketches differed: from the two proposals selected for 3D mock-up-building, one was developed concertedly and one as an aside of the shared process. The latter development appeared to be independent, seeking little input from team members. All in all, three qualitatively different interactive design processes were identified, and quantitative levels of collaboration were measured through Convergent and Divergent Collaboration Indexes. To determine the role of sketches for each of the students' designing, gaze data was indispensable.