There a quite a few different ways to do it, and quite a few programs that can achieve a similar effect. You can do it manually in gimp/photoshop by stroking lines and drawing gradients (but that takes ages). You can also play around with the settings in more new-user friendly programs like DA Muro or Flame Painter. Either way after a bit of googling I'm sure you can find the guide or program that suits you best. I use Flame Painter.