Synthesis writing is a cognitively and linguistically demanding task, and even more so in a foreign language. Collaboration can help students to learn how to elaborate the content of the source texts and integrate it in a new coherent text. But to be effective, online collaboration has to be pedagogically supported. In this chapter, a promising approach to support students in this complex task is presented and evidence for its effectiveness is provided. The effect of two instructional support instruments, a video-based model and a script, on the development of collaborative writing processes is described. Using a mixed-methods approach, collaboration intensity and efficiency are measured quantitatively, and collaboration effectiveness is observed qualitatively. The data originate from an in-class intervention study: 42 university college students grouped in triads wrote three syntheses in subsequent sessions, collaborating in Google Docs, and being supported in two of the sessions. The analysis results confirm the benefit of the support instruments used and broaden the understanding of collaborative writing processes: qualitative insight shows that joint information processing is intensified through scaffolding, and leads to a better content selection and to good peer-induced revisions of the jointly produced text on different levels. Regarding of collaboration efficiency, work flow was improved through scaffolding, albeit to a less noticeable degree for the high-proficiency groups.