There are a couple of options for implementing workflow in iDempiere:
Traditional workflow as described below. I typically do not use this option because it is to rigid and cumbersome. The real world needs a much more fluid and user friendly option.
Request - This is the option I use 90% of the time. Most people know what group/role or what person they need for approval/action.
Queue based workflow - where the natural process of acting performing a task naturally puts a record into the queue of the next worker. This is made possible because iDempiere can expose the results of a database view in a window. A given window can be defined to show all records requiring a given step. Pick queues in fulfillment work like this. A sales person completes the order of a stocked item. The completed order automatically gets picked up in a queue by a database view as being ready for pick. Once the item is picked, the next pack queue see the record, etc...
Project and Project Manufacturing - you can use the Project window => Phase subtab => Project Line architecture to group actions into logical phases that can be implemented in serial or parallel.
External workflow - dedicated workflow tools like Camunda can help you orchestrate complex tasks and navigate complete decision tree. While I have not implemented Camunda with iDempiere. The concept is very appealing. This is especially true for shop floor manufacturing where you bill of material counts are lower; however, your routes are intensely complicated.
The purpose of the remaining page is to help you understand how to use and manipulate iDempiere traditional workflow. The best resource for understanding workflow comes in the ADempiere 3.4 Solutions book. There is an entire dedicated chapter to workflow. Buying the e-book version of this book is worth it just for the workflow section. Below are examples.
2014-11-06 Using Workflow to Create an Approval Process for Physical Inventory
Introduction
How to create approval process for Material Receipt, Shipment (Customer), Inventory Move
0:01:45 Demonstration of what I created for approving a Physical Inventory
0:03:30 Why I did not use the existing isApproved field as a Workflow=>Node=>Document Action of "User Choice".
0:06:30 How to create the two new columns on the Physical Inventory window.
0:07:45 Modify the existing workflow using the Workflow Editor