Bulb mode
Flash fire, make sure if you have someone running the fire, the person is not in the frame
Paint fire,
stop bulb mode
The key is to have super powerful strobe because you want the ambient light super low. If your flash isnt that powerful, then you will mix too much ambient light and you will see the person holding the torch. I dont think you can do this with just a speedlite. You may need several of them. I used AB1600 bare at like 1/2 power.