1. For this tutorial your going to need a good stock photo lined up. Something with action… and preferably dark (this effect works much better in a dark setting)
2. Open the stock photo, then go to the paths tab and click create new path.
Press "P" on the keyboard to select the pen tool, Get a basic idea of where the fire is going. Keep in mind dimension and action. Swirls do wonders!
3. Click to start your path.
Click your next point, hold and drag to create a bezier spline path, which is necessary to create flowing lines.
Continue doing this, following the idea of where you want the fire to go.
Don't worry about getting it right the first time as you can change the spline at any time… another little tip - if you press and hold ctrl while using the pen tool you can temporarily access the direct selection tool which will allow you to edit the spline (a big time saver)
4. And eventually… you get something like what i have below. Remember that your only going to use the parts of your path that are in front of the object your path follows… in this case the models arm, and leg. So don't waste time trying to make the hidden parts look pretty.
5. Now you want to select a soft round brush (i used a 5 pixel brush here) and pick an orange color.
Make a new layer and name it Fire. Go back to the paths tab and select stroke path with brush.
6. You should immediately see the stroked path.
On mine you'll see splatters and flairs that weren't part of the original path. Those give the fire line the illusion that it is moving, and were accomplished by simply grabbing the clone stamp tool - using a spatter brush and cloning/stamping areas of the path at random.
7. Mask/erase the parts of the path that are behind the legs/arms or whatever it is your paths follows.
8. Right click on the Fire layer and select blending options, and use these options/values.
Feel free to play with the settings (that's how i discovered this technique).
You might end up with something better!
9. Duplicate the Fire layer, and set the layer mode to Overaly.
Click the eye next to inner shadow and inner glow on the duplicate layer to deactivate them.
10. Now double click on outer glow on the duplicate and change the values to this.
Spend a little time, and you can make something like what i have below…
For a different effect, duplicate the Fire layer again, and filter>distort>wave.
Play with the opacity levels, and play with the blending modes - particularly Overlay.