This is one of the exercises from my video course I am putting together. The download version will have instructional audio that explains how to achieve this level of confidence in your line drawing.
By the way, about a month ago I clicked something in the settings section of this blog and it disabled comments. Not sure what I did but after I tried to undo it by clicking "enable comments," the comments still seem to be disabled. If anyone knows what I could do to fix this I would appreciate it if you emailed me with your input. Thanks and sorry about that.