There is an easy, fast way to blend fire into any scene. This won't work in some cases but on average is a nice method to make a basic and realistic blending. Here, we will create a body out of fire. This is our base composition:
What you need to do is to place your favorite fire photos (preferably on dark/black backgrounds) on Screen blending mode. At first sight there's something missing, and that is the dark areas of the fire, which are muted due to our blending mode.
Create a new layer between the fire layers and your subject and use the Brush tool with medium Opacity and Flow and black color (a dark brown/orange might work as well, depending on your background and color settings) to bring back the dark parts of the fire. Use the Eraser to get rid of any strokes that go beyond the fire itself. A pen tablet is highly recommended to make this process more accurate.
Create another layer, pick an orange tone of your choice, similar to the midtones of the fire, and create a radial foreground to transparent gradient, parting from the center of the fire to the opposite corner, and then use again Soft Light to tint your new lighting.
Don’t forget to paint the light reflections on the model as well as adding sparks, particles, and other cool stuff that will bring our fire to life. Again, it will be easier if you have them photographed on dark background to use the Screen blending mode. This could be our final result: