Its pretty straight forward.I can't do a whole tutorial but I can show you a quick example.

Press 1 on the numpad, then add a 32x32 UV sphere. This will be our "original mesh".

Tab back into object mode, and add a plane. Angle your view to the side a little and press G,Y and move the plane in front of the sphere.

Now press 1 on the numpad again and subdivide the plane a few times.

Now click the retopo button, in the mesh panel.

Now the plane should be plastered to the sphere like so! You might have to Grab the plane then release to get it to update.

It totally depends on the angle you are viewing from. It uses the z-buffer for speed. Now as you move the plane around it will deform to the sphere in real-time.

Let me make some more pics and Ill do a little retopo paint example if you want.