I am following the step-by-step guide to creating a custom workflow.

The anwser to my question, regarding how to get / where to place a new custom-workflow-context.xml file got answered in this very usefull forum topic –

I am once again a little frustrated with the Alfresco system.
Why? The “hello world” workflow won’t work… The simplest.xml workflow didn’t work.
And “A system error happened during the operation: null”  is not really a good error in my humble opinion.