Creating realistic sheets of fire is a lot harder than setting objects
on fire. This is the closest I've been able to get to something
that works. If you have suggestions or refinements,
please tell me.
You will want to do this in a very big image. I'm showing a reduced
size here.
Set your foreground and background colors to yellow and dark orange.
In a layer (don't use the background for this),
render clouds.
Add a layer above this, and reset the colors to black and white.
Render another cloud, and set the layer attribute to "hard light."
Merge these two layers. Select all, then choose layer -> free
transform. Give the flames a horizontal compression of about a
third.
Load the transparency mask as a selection, then choose layer ->
transform -> skew. Drag the middle handle at the top to one
side until the flames show a distinct slant.
With the cropping tool, select a rectangular area that you will use
for your final image.
Duplicate this layer, and give the copy a large ripple, with amount
set to about 100.
Use image -> adjust -> brightness/contrast to increase brightness of the top layer to +50. Set the layer attribute to
"multiply."
If you need something darker or more malevolent,
go to the lower flame layer, and use image -> adjust ->
brightness/contrast again and adjust both brightness and contrast
to taste.
That's about where it stands for now. This works when you need an area of fire, such as the one I used in the Last Supper.