I do flames+firebolt for sustain ... flames debuff target that gets hit by fireball after for extra damage. Two firebolts/ice spikes against melee when I need burst. Two lightning bolts against mages till they run out of magic.
This is not what "extra damage" means. It's poorly worded, I know.
Flames applies a damage-over-time effect onto targets that it hits. This effect does not stack, and is refreshed with every tick. Basically, it does nothing until the flames spell expires. Once you stop casting, the target will take minor damage over time.
Firebolt works in a similar way, only since it's not channeled, you're guaranteed some DoT ticks. The DoT doesn't stack though, so chaincasting firebolt will overwrite the previous one's DoT.
Basically it doesn't increase the damage that the target takes, but applies a non-stacking DoT after the spell hits.