Testing a flow involves triggering it (e.g. by adding a record to the flow data source) and following it as it moves from step-to-step.

Bear in mind that only the user/group/role, etc. assigned to the step is given the link to proceed the flow, you must be logged in as one of the users responsible for the step.

This can be tedious if you do it manually, so the best way to do this is to assign yourself as the user for each swimlane.