When you start a workflow method with WAFContext.Session.StartWorkflowMethod(wf), it returns a WorkflowInfo object. The WorkflowInfo object contains two properties you need to use: InstanceId and PercentageCompleted.
This combined with WAFContext.Session.GetWorkflowInfo(Guid instanceId) which you can use to look up the progress with each poll request, should get you going.
Also note that this is dependent on you setting the progress inside the workflow using WFContext.SetProgress.
Let us know how it works for you!