Use iMovie 09 for Green Screen effects
A step by step guide on how to create excellent videos with the green screen effect. You will need to construct a green screen, for more help go here.For this tutorial you will need two video clips, one that has been recorded with a green screen background and one that is the background you would like to change to.
Step one: You will need to start a New Project with the ⌘N, then choose a Project Name, Aspect Ratio (Widescreen 16:9), and for Theme, choose None. Now just click the Create button.
You will see a blank project. Select Import the File Menu
Select Movies, from the Import submenu…
Step three: Choose the two files from your hard disk and click the Import Button. The two video files will be imported to your project.
Step four: Click Preferences… from the iMovie menu. From the General tab of the iMovie Preferences window check the Show Advanced Tools box then close the window.
Drag the selected background clip from the bottom event window to the project timeline at the top.
Step five: Next select a portion of the green screen video you would like to overlay on your background clip by clicking and then dragging the mouse pointer from the start to the end of the desired clip.
Drag this clip from the bottom event window and drop it on top of the background clip already in your timeline.
Select Green Screen from the popup menu that appears.
Step six: Your green screen footage will now automatically be overlayed on the background clip. Click the SpaceBar to play the video from the current location.
Step seven: If your object is stationary or the green screen didn’t quite make it to the edges of the video clip you can crop the area used to green screen.
Select the green screen footage from the timeline then click the Cropped button in the video preview window.
Drag the four corner handles to frame the clip as desired then click the Done button.
Green Screen [via iClarified]