Bend the rotor arms not the disc surface.
Not possible on the two-piece rotors. Furthermore, if the carrier is bent on the two-piece rotors, it may not be possible to get them back to true.
It’s a job best done on the bike, I think. If the rotor was already on the bike and in a working brake, then the pads can be used to center – just spin the wheel and take note of where it goes ‘ching’. Back the rotor out of the pads and use either a proper truing tool or, as already suggested, an adjustable wrench. To bend a rotor back to true, you need to bend it past the point of true so that when relaxes again it is in the correct place. It’s a really simple process once you’ve sussed it out – so much so that it can be done without the use of tools if necessary (clean hands or gloves, obviously).