We freeze branded for the first time in late July. I intended to use nitrogen, but found out at the last minute you can't have it put in anything but a certified cryogenic cooler. So I had to switch to dry ice.
ZNT helped us. Neither of us had done it before. I had never seen it done. We branded 50 head. It seems to have worked great. The brands left a scar (probably not the correct term) that was readable a couple of days afterwards, the skin peeled, and now white hair is already coming back in. They are very clear.
Dry ice is plenty cold enough, the whole key is the solvent. Apparently most people use alcohol - it needs to be almost pure. However, alcohol will absorb moisture out of the air and quit working. Others use gasoline, but I was worried about odor and the explosiveness. I found one reference that suggested acetone. So, I went to the paint store and bought 5 gallons of it. It worked great. No odor, and not nearly as dangerous as gasoline. It was very wet the day we did it and we didn't have any problems.
Wasn't hard at all.