I just did this 5 minutes ago for the first time, and read this thread first. Here is my experience:
I had a suction mount vise similar to this:
Perfect for holding a head without damaging it. I then used a cordless drill and started with a small drill bit and worked my way up. I had a TTM19 body minus the head to use as a test.
I started by drilling test holes into a block of wood. I found that a 3/8" drill bit was a little too small, and the next one up, a 1/2" was too big. With the 1/2" bit, the neck post went right in with room to spare all the way around. So if I used that size, my head would flop all over the place. I believe there is some sort of putty that people use, so if you plan on using that, then drilling the hole too big probably wouldn't be a big deal. I wanted a snug fit between my neck post and my head so I went with the 3/8" bit.
So when I drilled into my resin head, I started pretty small and worked my way up to the 3/8" bit. After drilling a straight 3/8" hole, I then slowly rotated the drill around just a little to slightly widen the hole. Then I got out my Dremmel and used one of the pointed abrasive bits to gradually increase the opening and smooth the inside out.
As mentioned, take your time. And make sure your initial hole is centered.
Oh one more thing. The big 3/8" drill bit grabbed quite a bit more than the smaller ones because it's so big. If I had been going too fast, I may have drilled it right through the other side of my head! so GO SLOW!
Hope this helps someone.