Try to build a fire using an emitter and after you're sattisfied with how it looks in place add and animate a Path Wind. That should do the trick. After that you can add other efects... the sky's the limit
By the way the "X" in Zorro was actually a "Z"
OR You could edit the letter or whatever contour you want to light up in Modeler using splines. Before you finalize editing, and after the final shape tweaking you just have to max out their points by hiting n and increasing the divisions (I think 200 is max).
Then in Layout you ad an emitter to that object and from the nozzle chose object vertices.
A more detailed description is given in the growing vines tutorial but I can't remember where I saw it.