This part was the hardest for me because I didn't have anything to hold the ring while I drilled it. You need to hold the ring in something that won't scratch it. A vice or vice grips. What ever you use, make sure you pad it to protect the ring. You don't want all that tapping to go to waste. **Also note that drilling the ring gets it very hot. It can burn you, handle with care**
I drilled a hole in the center followed by a bigger hole and so on. Drill out as much of the center as you can. A dremel will probably work well here. I didn't have one for my first ring. Once you have enough of the center out you can grind away the edges with a file or dremel.
At this point you can play around with the size. Try the ring on and grind away until it fits right.