I didn't do a tutorial, but since Acer mentioned my name in vain, lol....
That can be a study, to say the least. I've found that it helps to use a very sharp, very thin tool to make the initial cut in the wood. I don't go deep at first, just enough to cut the wood fibers near the surface. After that I use a small screw driver that I ground down to a slim profile to deepen the cut. By small, I mean about an 1/8" wide. Slim, but not so slim that you worry about breaking it in the cut. I then gently open the cut with a thicker skew to accept the wire.
It's more problematic when you go perpendicular to the grain, not as much when you run parallel to the grain, so that can help when you're laying out the wire design.
If I get a chance over the weekend, I'll try to take some photos... Hope that helps some.
Ed