It's a mystery to me to how they really did it. (though I didnt make any effort to investigate either) I suppose just moving along the bearing was simple to do.
But I myself made a tool that allows you to calculate course/aob with three bearings only if you are not moving. As I said, I have no idea if the germans or even the americans used this technique during the war. Just that it can be done with enough patience.
http://www.subsim.com/radioroom/showthread.php?t=147719
Read also:
http://www.subsim.com/radioroom/showthread.php?t=110619