Well it's really easy to do!
1)Create a new layer
2)Go to Filter->render->clouds
3)Use the brush tool to clear some parts
4)Lower the opacity to 20% and start darkning areas using the brush tool to make it look realer!
5)Go to Layer->new adjustments layer->Gradient Map. In the gradient map window tick the dither checkbox. In the gradient editor choose black, white gradient color(those little squares). Add two new color stops. The first one from the left will be black, the second orange, the third yellow and the last white. Play with the color stops a little till you get a nice settesfing image!
And you have created nice flames!