Drilling the first hole was slightly nerve-wracking. It isn't exactly obvious where to drill, and I didn't want to miss the gas passage. Feeling the drill bit break through the wall and into the passage was VERY satisfying...Start with a small drill bit and work your way up. The last bit size that I used was a #36. I used a 6-32 tap to cut the threads.
Unfortunately, I broke off the end of the first tap in the gas block.
I used an inverted can of computer duster gas to freeze the tap, and then I hit it with a center punch to shatter it. That sort of worked, but most of the tap ended up deeper in the gas block, so I had to disassemble everything I did on that barrel up to that point to remove the gas block and fish out the rest of the tap.
You will either need to slightly overdrill the hole or use a bottoming tap to finish cutting the threads, since the tapered point on a standard 6-32 tap will bottom out before the threads are cut all the way through.