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Open a new file, I used 400×400, press D to reset your colors.
Goto Filter > Render > Clouds,
then Filter > Render > Difference clouds
keep pressing Ctrl+F until you get something that's sort of like fire such as:
Then use the brush tool with the color black to go over parts at the top to make it look like the fire ends somewhere and doesn't just take up the whole image such as:
Then go to Filter > Liquefy
using a size 15 brush, with 50-60 density, 100 brush pressure to sharpen the ends and curve them to a more fire like effect.
and there you have it, a fire like effect, simply… it's all down to the difference clouds really…
Let's try to use it on a render.
I used this image:
Have your image and fireback next to each other.
Hold down your left mouse button and drag your fire onto the other image.
Now set your top layer mode to Screen.
My image came out looking like this.