Step 1 Make a new image with dimensions 1000x500
Draw a gradiant like so (with white at the bottom and black at the top):

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Step 2 Make a new layer.
Double-click Layer 2 to edit its properties
Set the Blending Mode to 'Overlay', then hit OK.

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Step 3 Go to the 'Effects' menu and choose 'Clouds'. Change the blending mode to 'Difference', then hit OK.

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Step 4 Push CTRL+F to repeat the effect. Do this about a dozen times.

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Step 5 On the 'Image' menu, choose 'Flatten'. This will give you a single layer with some nice black-and-white clouds.
On the 'Adjustments' menu, choose 'Curves...'.
Choose 'RGB' under 'Transfer map', then uncheck the 'Red' and 'Green' checkboxes so you're only modifying the blue channel, add a control point as shown below. Do the same for the green channel so you have the curves shown below.

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The final result being:

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Other things to try:
- Use different shapes for the Layer 1 Gradiant. Round gradients make neat looking explosions
- Tweak with the Curves, the general shape of them should be preserved to keep good saturation, but the channels don't need to go in the above order. Try Green, Red, Blue (instead of Red, Green, Blue) for a copper flame (copper burns green).
- Play with the 'scale' setting of Clouds for finer or Coarser clouds
- Play with the Roughness setting for clouds for softer or sharper fire
- Just try things out, you never know what you'll get.