How to debug the workflow from the portal?

Hi Experts,

I have a requirement to expand the existing workflow.I have included the new steps in workflow.But I cannot able to debug the the task.I have assigned a class method to this task.I have put the external break point in that method ,but it is not triggering.Kindly any one adivse how to debugg workflow from portal.

Thanks in advance,

Anusha.B