Light painting with fire - Taking the pictures
Once you have everything you need, you can finally start the fun part: Playing with fire and taking pictures.
That said: BE SAFE.
Camera Settings and Setup
Set up your camera(s) on a tripod where you want them, and make sure your trigger or shutter release are working. Once they're set up, try to touch/move them as little as possible so that it's easy to combine pictures later using Photoshop. You'll probably have to check the settings for a few test shots, but try not to touch them after that.
ND 8 filter (I think)
ND 4 filter (I think)
You can see above some different camera settings that I've used. I recommend putting a weak ND (neutral density) filter on your lens, ND-2 or ND-4. The fire is slightly too bright for the camera when the lens is at minimum aperture, and even a weak ND filter helps your highlights not be blown out.
I think I also set the white balance to something other than auto, so that all the pictures would look the same and so that editing them would be a little easier. You can use your test shots to help decide which setting works best for you.
Don't pay too much attention to the exposure time of the pictures. That's just how long it took to move the fire across the frame, from my starting the exposure with a remote trigger to the time that I end it.
Bulb mode - You'll have to have the camera in manual mode, with the shutter speed set to BULB in order to be able to control how long the shutter is open using a trigger or shutter release cable.
If you don't have a good driveway to do these in, it can be hard to find a good location. There are a few things we need
Darkness - it needs to be a pretty dark area, so the fire and the subject are all that you see in the picture. I've taken the pictures with a streetlight nearby, and that's fine, but sometimes you can see a reflection of it on the car.
Nothing that will burn - Don't do this somewhere with dry grass. Some drops of burning fuel will inevitably fall on the ground, and then it could be bad.
Legal - Fire is very bright, and people are generally uncomfortable when they see people playing with fire (it kinda looks like a cult thing), and might call the cops if they see you. I had the cops called on my when I was doing it in a school parking lot once, and the cops were friendly, but still asked us to leave. If you do talk to the cops, just be friendly, explain it's for photography, and maybe show them the pictures on the camera screen.
Setup - where to put everything
The above pictures is a top view of how I set up everything for the photo shoot. Starting at the camera on the right, you see we have two cameras at different positions around the car. One of them has someone controlling the exposures, and I guess it would have a wireless trigger to talk to the other camera.
Near the bottom left we have the bucket of Coleman's camp fuel to soak the Kevlar wick in.
A few feet away from the bucket of flammable liquid we have the lighter. I usually light the wick, then just drop the lighter and find it later.
The dotted blue line represents the path that I walk for a single picture. You can usually walk around several times (about 30 seconds) from one soaking of the wick.
On the top right we have the towel that is soaked with water.
You don't have to set it up this way, this is just what I have done. No matter how you do it though, make sure your fuel, lighter, and extinguishing towel are all far away from each other.
Soak the Kevlar fire wick in Coleman's fuel for 10 seconds or more.
Remove it from the fuel, attach it to your stick.
Standing multiple feet away from the bucket of fuel, light the Kevlar fire wick on fire.
Start the camera taking a long-exposure picture
Walk across the frame with the fire
Stop/start the camera as many times as you can until the fire dies down a little
Walk over to the soaked towel, drop/coil the fire wick on one half of the towel
Extinguish the flames by folding the other half of the towel over the flames.
Video demonstration of the process of taking the pictures. Unfortunately, the video stops right before I coil it onto the towel and extinguish the flames.