The first thing you do is make a new image of any size. Using any orange related color, you make what looks like an orange blob.  You must make sure that the image has black spots around its interior.

For personal preferences you might consider adding small specs of red, after all fire does have some red in it. All the brushes used should have an anti-alias hint to them, so the edges don't look too hard.  Your fire should look smooth and beautiful, like poetry.

Now the first real step towards building your very own fire is to click the Filter menu, and run your mouse over the Distort topic which shows another menu in which you click on ripple.  You click and drag the slider deal all the way to the left/right, and keep the size at Medium.   Now click OK.

Now here is where you decide what you want you fire to look like, of course you ripple a couple more times. However many times you decide is fine.  The more you ripple the more it blends and looks harder.

7 times, including the first--------->

All you do for now is decide whether or not you like your fire.  If not you can do things like add more black, and ripple, or even add more orange.  You can definitly do anything you like.

<---------------Added more color, then rippled it again.

Now this step would probably be just touching it up, maybe ripple it some more, or color dodge/burn it. This help make the fire look more real, so if you not looking for semi-serious fire then don't use the dodge/burn tool. You can also use a brush of your choice or copy and paste different parts of the image around the picture.**Remember there is no distinct shape or movement of fire**

Now this is usually the last step, but it doesn't have to be. I always end my fire with a good blur, or random filters which give it a hint of blur.  Sometimes fire doesn't look good with blur, in which you leave it alone.  The blur is only a touch up tool as well.

There is an example of some fire I have done in the past.