This trick will help you blend a fire stock to fit the colors and the atmosphere of your photomanipulation.
To do so, we are going to use Alpha Channels and Adjustment Layers.
To understand this tip you should already know how to create layers and how to use masks.
The first step will be to remove the black background that comes with most fire stocks you will find.
Some of you may already know you can apply blending modes to your layer to make that black background disappear.
It's the easiest and fastest way to blend a fire... but with this technique, you won't be able to modify your fire's aspect (or your background will re-appear) ! So your result won't always look very realistic.
That's why I'll tell you how to cut (softly) that black background, using the Alpha Channels :
First, display your fire stock layer only (so the rest of your image won't bother you) and search for your "Channels" window (in Photoshop CS6, there is a "Channel" tab right next to your "Layers" tab).
You'll see at least 4 layers : 1 for each color that composes your image and the one with all 3 colors together (the RGB channel). By default, the RGB channel is selected. Select one by one your Red, Green and Blue Channel and determine on which one there is more contrast between the fire and background.
Copy the Channel you chose and adjust the contrast and luminosity to make the background darker and the fire lighter. Don't forget to work on the copy of your channel, never work on the original one or it will modify your original image !
Consider your channel like a mask, we'll use it to get a soft selection of your fire : all the parts you'll want to disappear need to be black and all the parts you want to display need to be white.
Once you think your contrast is ideal to keep your fire and make your background disappear, get the selection corresponding to your channel by hitting ctrl and clicking on your channel's thumb. Select you RGB channel again to restore the colors. Go back to your layers and use the currently active selection to create a mask for your fire layer.
If you still have pieces of background left or if you erased some parts of your fire you wanted to keep, you can go back to edit your channel until you get a satisfying result.
Now that you erased your background, you can edit your fire's aspect :
The best way to do it is to use Adjustment Layers, because they will allow you to edit the settings over and over again, without loosing quality.
Make sure you always get back the selection of your fire layer (hit ctrl and click on the mask's thumbnail) before you create an adjustment layer of any kind. Like that you will create for each layer, a mask that will be identical to your fire's.
To proceed, click on the little icon that is between the mask's icon and the group of layer's icon (below your layers in Photoshop CS6) and select the kind of adjustment you want to apply to your fire.
Click on that new layer to adjust the settings, you can create as many layers as you want (Or as many as your computer can handle !
). Play with the color balance, the luminosity, contrast, saturation...etc until you feel like your fire really seems to belong with your background.
These concepts might be a little difficult to understand at the beginning, but once you got them, you'll probably get addicted.
Using the channels to cut a background is also the best way to blend hairs cleanly and easily, once you master that technique you won't even need to use painted hair anymore !