Well the real person to ask, the expert on cool skies is Jon Mikal. This is just my feeble attempt to try it out for myself.
Here is what I did: I took a desaturated sky, made a duplicate layer, selected only the sky, burned it, threw a sepia filter on it and then adjusted the opacity and smashed the layer in.
I'd really love to here from Jon Mikal about how he does it though. I was just winging it.....