There's a few flame effect tutorials out there, but all the ones I've found are either badly outdated or too fiddly. This is how I set any object - including text - ablaze with the Gimp, in just six quick steps.
Done with Gimp 2.2.13, which I'll keep using for tutorials until more Linux distros start including 2.4 by default. If you've soldiered on to 2.4, the menus will be all switched around, but the effects should all still be there somewhere. Click images to get full-size shot.