Although SGTM's way is alot better, this way is the simple option, i found this becuase i couldn't get SGTM's way to work correctly,

Basicly you follow the first part of SGTM's method.

Start with a blank image say 600x600


Once in press the key command "Ctrl + I" to invert the background colour to black

Next enter what ever text you would like to have flames, Center this or place it in what ever postion you want as it can't be moved or altered after you proced.

Rotate the canvas CW "Edit-->Rotate Canvas-->Rotate 90 CW"
right click the text layer and "Rasterize Type"


Next we need to add the begginnings of the flames:
Click "Filter-->Stylize-->Wind" Repeat this process 3 times or until you get the right hight for you. Tip: Press "Ctrl + F" to repeat the filter

then rotate the canvas CCW back upright.

Now select "Filter-->Blur-->Gaussian Blur" and use setting of between 2 and 4


now there are various evvefts you can use here, you can use the ripple from the distort menu, or you can use the Liquidfy option, which is what i will use here.

Merge the layers into one, including the background:


now select the liquidfy option "Filter-->Liquidfy" Use the swirl, warp and distort options to create what you think looks like fire.

Once finished select:
"Image-->Mode-->Grayscale"
Then "Image-->Mode-->Indexed Colour"
then "Image-->Mode-->Colour Table"
From within the new window select Black body


and now the finished product:


If you kept a copy of the text you can even place the text ontop in black or even make a reflection.

All in all, this is a good effect, but SGTM's method give's you better control over the colour of the fire, this is just easier if you couldn't figure out the other methods.