I'll give you an idea.
Get a skull...a model of a skull which can easily be bourght or borrowed. Place it against either a black screen or bluescreen, perferably black seeing as this will not reflect on the skull.
Next film or take a photo of your skull at carious angles, now it's compositing time. Remove the background with keys if it's blue or change the composite mode to 'screen' oto remove black. Add some stock footage of fire onto the skull, do some masking to make the fire look like it's there, finally color correct it all to green, Bingo.should work