How do I add Green screen effects in iMovie 10.0.6

I have been working on a project and have green screen elements as well as pr keyed elements that I want to add to my film but am unable to figure out ow to do it. How do I add Green screen effects in iMovie 10.0.6?
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Have a look at http://help.apple.com/imovie/mac/10.0/#mov82da1338a
Geoff.