Subgoal labeling is a technique known to support learning new knowledge by clustering a group of steps into a higher-level conceptual unit. It has been shown to improve learning by helping learners to form the right mental model. While many learners view video tutorials nowadays, subgoal labels are often not available unless manually provided at production time. This work addresses the challenge of collecting and presenting subgoal labels to a large number of video tutorials. We introduce a mixed-initiative approach to collect subgoal labels in a scalable and efficient manner. The key component of this method is learnersourcing, which channels learners' activities using the video interface into useful input to the system. The presented method will contribute to the broader availability of subgoal labels in how-to videos.