Step 1
Open a new file I used 400x400, press D to reset your colours.

Step 2
Goto Filter > Render> Clouds

Step 3
 then Filter > Render > Difference clouds 

Step 4
keep pressing Ctrl+F until you get something thats sort of like fire such as this.

Step 5
 Then use the brush tool 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 this.

Step 6
 Then goto Filter > Liquify 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

Step 7
Then goto Image > Adjustments > Gradient Map.  Try to make the gradient look something like this.

Step 8
The final product should look something like this.  You may wish to take more time with the liquify tool to give it a more realistic looking flame effect.  This is just a rough version to show you what it would be like.