Or download the free VC Reflect from VideoCoPilot. It is made for reflections ofcourse but can also be used for shadows by colorizing the reflection.
Be aware that the plugin has a few nasty bugs that sometimes wont render frames if certain conditions are not what the plugin likes . experienced it both with comps with no pixeldata and certain skew values in the plugin.
else i would follow what Mylenium says.
Robert