"Quickly" must be a relative term. Steps 3 & 4 would take you about 10-15 min on a really complex pic like they use in the example (hair is always hard to do with a pen tool).
Mask Pro does not support the GIMP, unless I missed something. There is a faster way to do what I describe in the article. The link is in the comments.
PS built-in tool and Mask Pro (http://www.ononesoftware.com/detail.php?prodLine_id=4 ) are the best and fastest ways to do this. You can do this task above in about 1 min with the right tool.