Given that the core approach could potentially be carried out using a paper and pencil, I met with my friend (and fellow indiehacker - @zhak) and tried it out on one of his projects.
I asked simple questions such as "What milestones have you recently achieved" and drew them as circles at the bottom of the paper, then asked "What goals do you ultimately want to achieve?" and drew them as circles at the top of the paper. We then wrote down high level steps in between and drew arrows ONLY where completing a task / reaching a milestone lead contributed to the realization of the circle that the arrow pointed to.
After adding in all missing intermediate steps, and thinking about alternative ways to get to the ultimate goals, we had (a very messy one, but one indeed) the first user-tested GoalCanvas.