Also, the thicker the ice and the more opaque it is, the less sunlight will penetrate the ice. In other words, if there's 12"-18" of ice that is very milky in color, I would err on the side of caution and fire up the compressor and open up the pond. I'd actually probably fire it up sooner than that, but just using that as an example. A couple dollars of electricity vs. the dollar amount of fish in the pond is a cheap insurance policy in my mind.
I pretty much do it the way Esshup describes. The only thing I will add is I try to blow the hole on a "warmer" day if possible. My thinking is when temps are really cold, that snow/ice cover is acting as insulation and keeping the water a little warmer maybe. If there is going to be a warm up in the next few days, I wait till then to blow the hole. Also, the air you are pumping into the pond is warmer on a warmer day so not chilling the water as much as pumping super cold air.
Just my 2 cents
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