Here are a few animated pictures showing a bit how some pros hit a topspin.
It could be a good illustration of how to hit the ball at the contact with your racket, although the complete swing is not always shown. The key for a heavy topspin is certainly to figure that you have the racket head speed while brushing the ball with an upwards motion from -to simplify- low toto high.
Also on youtube.com I have already seen some very good and detailed super slow motion videos of Federer's forehands. Go there, try to search for them with some key terms and see if you find them. Maybe you also find a topspin stroke being showed in one of them.