Create your mask or your shape or whatever it is you want to follow the person.
create a new video layer and put this object in it, make sure its on top of the video you are following.
now, double click the shape and it opens in the canvas (left viewing window)
click the tab at the top that says basic motion I believe (something like that, as I dont have FCP on this machine to look)
and inside are a bunch of parameters you can change. One should be X/Y position or something like that.
in the viewer (right viewing window) there is a tab that says view I believe, and change it to "image+wireframe". now whatever you have selected in the timeline will be highlited in the canvas.
click and move your shape around in the canvas, and notice the #'s change in the basic motion info of the shape.
find the first frame of your player, set the playhead there, in the main timeline.
double click the shape again, and it opens again in the left viewer. Go back to basic motion. position the shape (with its wireframe now visible) by clicking and dragging the shape to where you want it on that first frame in the right viewer.
inside the basic motion tab click the diamond shape next to the paramater and a black diamond "keyframe" is created.
now, go to the next frame and repeat this for EVERY single frame you want to be followed. you may be able to get away with every other frame, or even larger gaps in the keyframes if the object to be followed doesnt move much.
hope that helps.