Here is a tutorial ( not very good but you get the basics ) with pictures
Select the footage you want, but select it twice.
then drag each one across to the window
this will give you 2 layers, and will show up down the bottom as 2 layershttp://sg.toson.net/a1.jpg
now you need to add a mask around the top layer ( the footage )
drag the edges of the mask to go around the person you need.
Because the picture ( layer2 ) is the same as layer 1, this can be a little tricky getting the mask around your person, if the mask has not got enough dot ( move points ) you can add more maskshttp://sg.toson.net/a2.jpg
Next you need to select the time ( thingy )
to do this
click on the layer at the bottom of the screen where the title is.
you will see an arrow shape, click on it, and it will push down some more bits, the bits will say transform and masks
now click the mask arrow and it will bring down the mask features, there are little circles, select them all, this will now allow you to move the timeline and the mask will move ( if you change it per frame ) if you don't click on the little circles (clocks) then each frame the mask will stay the same.http://sg.toson.net/a3.jpg
Ok now select the effect you want, ( the same clocks apply for transform and any effects, so you can move them over time. )
now just move along frame by frame adjusting the mask around the actor.http://sg.toson.net/a4.jpg
if anything is unclear let me know ( proberly all of it