This is the way I am now doing my slotted heads with a jig I made:
The jig is pretty simple-just some scraps of wood and polycarbonate (Lexan)and a couple of toggle clamps.
The first step is to drill the tuning machine holes.
I first mark the centers out on one side and then transfer the holes to the other side by drawing perpendicular lines across the head:
Next,I use a Stanley doweling jig to drill the holes:
Notice that the drill bushing is a piece of brass plumbing pipe.You need to custom make a bushing in order to get the exact size drill bit for the tuning machines.
I mark the bit with tape in order to stop at the correct depth.
Then I simply line up the index mark on the jig with my reference lines and drill the holes:
Next, I mark the slots and cut out the bulk of the wood with a jig saw:
Here's the jig:
I clamp it to the head:
I then install the proper bushing on the router base-notice that I write down the size of the router bit and bushing on the jig for future reference.(I think the last time I used this jig was 4 years ago!)
I route the slot in 3 passes. A plunge router would be better for this,but I don't have one (yet!-I have every other tool known to man though!).
Here are the finished slots-all they need is a little clean up with some fine sandpaper.
It looks just like mine! 
