Here are three brief videos which show the shaping and finishing. When I say 'tile file' I actually mean 'tile saw'.
Note the rasp I'm using has a medium cut one side, fine the other and the edges are flat with no teeth at all so I can guide the edge of the rasp against my thumb without drawing blood!
The still pic show the nock 'finished' just for the purposes of this post, that is to say, I haven't shaped the groove to fit the actual string on the finished bow... well I can't 'cos I've only done one nock so far! The lip at the edge of the groove may be reduced and rounded a bit to make it all more subtle.
If you are wondering how I polish the string groove I use a needle file with some 240 grade wet & dry over it and then finally buff it up with an old bootlace which has been loaded with Vonax compound and is then worked back and forth diabolo fashion. If you check out the video, you'll notice the groove in the bock of Vonax where I've run the bootlace across it to load it with compound. This is my pet 'secret' technique so don't tell anyone ok? You can also just about make out the transition from flat back to rounded back as it approaches the nock.