I'm sure that a Flux pen might be easy to use, but how do you use Flux in packages like these (that's just the cheapest flux package available):

They come in a solid form, no spill, as they call it, and if you burn it with a soldering iron, I guess you used it up already. Maybe it's not designed for soldering irons? Although they mention soldering irons and all kinds of other things in the title.
Anyway, any experiences are welcome.