This is exactly what I did this year. I used a very fine pointed pair of surgical type scissors. I first candled the egg and marked a location w/o any veins. I then did it exactly as Justin described w/o any problems.
Last year I used a razor blade, and it didn't make as smooth of cuts and I ended up nicking several veins. They all survived just fine and were nice healthy hatchlings, but a couple of them got very messy/bloody.
The surgical scissor method is tops IMO.