Jake borrows the Flame Guitar from a citizen of the Fire Kingdom when he sings the song All Warmed Up Inside as a gift for the Flame King and Flame Princess, in the episode "Incendium." However, Flame King does not like the song and almost casts Jake out of the Fire Kingdom because of it. Flame Princess on the other hand seemed to enjoy the song.
The guitar seems to be made out of rack with flames leaking out of it. The top of it is covered with one large flame.