It can be tricky to add fire in a composite, especially if you don't use the appropriate technique. Here is how I do it:

1. Photos

• It is important to take photos of fire by yourself: doing so, you will be free to take as many pictures as you want, which will prove to be useful in the next steps.
• Wait for the night: you do need a black background behind the fire, it is not going to work well if the sky is not dark enough.
• Create your fire so that there is nothing directly behind it, again, to get a black background. Things can be there in the distance though, we don't really mind: they won't get enough light from the fire to really have any impact on the result. Regarding the fire itself, most of the time I use a cheap barbecue and some thin branches (I don't need the fire to last very long, a few minutes is enough).
• Shooting time! I usually use fully manual mode, with something like f/5.6, 1/250s, ISO400. If you need your picture to be brighter or darker, play with the aperture. Something between f/4 and f/7.1 should be fine whatsoever.
• I usually take about 200-300 photos, so that I end up with a good collection of various shapes.

2. Editing

• The key when editing fire, in my opinion, is to NOT change its shape very much. It can be tempting to use the 'liquify' tool a lot, but it will be hard to keep a realistic look. This is why I recommend to take so many photos after all: do spend the necessary time to pick the shapes that fit your project the best.
• Add the layer containing your selected fire photo over your project, and put the layer in 'screen' mode. The magic of blending modes will make anything black automatically disappear: no tedious masking work required!
• If needed, adjust the levels of your fire photo (I usually bump the blacks a bit, just to make sure the background behind the fire is fully pure black)
• Here you go!

This is the very technique I used in my latest creation, 'Burnout':

BurnoutAll the photos and editing by me (2014).

My wife is not feeling well these days. Usually, she's solid as a rock: working all day long, sometimes even during nights too, is quite normal for her. In fact, I rarely saw someone so passionate about their job. As a vet, she would never put her own comfort above the one of her furry patients, even when it implies staying unexpectedly for hours at her workplace, skipping sleep time, or letting an ill animal sleeping on the couch rather than in his cage. Observing her gives me the impression she's got many more arms than I do.

But about 6 weeks ago, things started to change. Her strengths started to leave her body. She lost much weight in two weeks, and she was not exactly fat to begin with. Staying on her feet became too hard, and she purely and simply had to stay in the bed. Working was simply unthinkable. Needless to say, I began to worry quite a lot: it was like everything in her was fragile. I was feeling the negativity trying to grasp both her body and her mind. One night, when she was trying to get some sleep despite her state, I took my tablet and started to draw a rough sketch of her. I knew the picture had to be dark, involving fire, consuming her, with things appearing from the darkness. When she saw the result in the morning, she liked it: according to her, while most of my sketches look like made by a 4 years old child, this one was much more convincing. So we decided to go for it, and shoot all the photos we needed. She bravely posed in our home studio. It was not easy for her: at the end of the shooting, she was very weak, and things started to revolve around her. I did my best to create an image from there. In the following days, we even discussed together about color adjustments, size of the flames, and various small details here and there. It was great to have some fun together.

Doctors say they can't do much for her. However, things should get much better in the following weeks. After all, even though the effects are very strong on her, they rarely last during the 9 months. Yes, she's pregnant, I am going to be a dad! :-)

Thanks for reading, and be careful with the fire!