1/ First print a markers (for example a L-marker here) or make your own L-marker by drawing it approximately on a sheet of paper, like this one (note, I put deliberately my finger over the edge to break it, else Silverlight will recognize the marker and put a "teapot" over it) :
and find a live demo, such as "Balder Sample".
3/ Then, you can allow Silverlight to have access to your WebCam and microphone :
4/ Start the fun by moving the marker around the screen : back and forth and spin it all over the screen. Make the teapot scale up and down, rotate or move it up and down.
If your did not yet have the opportunity to play with, I would recommend it. Moreover, your marker will work with other online Silverlight demos.