Problem:
I have to customize a TFS2010 Build and I don't know Windows Workflow 4.0.
Solution:
I watched the following video to quickly bring me up to speed on Windows Workflow 4.0 and then I read the workflow of the default build template.
http://msmvps.com/blogs/theproblemsolver/archive/2009/12/08/online-wf4-presentation-recordings.aspx