Best to use when
The Critical Path should be discussed after a Problem Statement has been agreed upon. Once completed, the Critical Path should give a step-by-step map of the user's most critical experience, from having the problem to solving the problem, and every step in-between.
Starting with the Problem Statement as the first step, as a group, use your understanding of the Problem Statement to map out the steps of the user's journey through solving that problem.
The Facilitator should stand at the whiteboard and draw the flow.
Keep adding steps until you've reached a solution.
Record all ideas in a separate document or Trello.