You start the reactor and keep it running and safe. Output the requested Power to earn points. Also you don't have to be very precise, the difference can be from -50 to 50 Megawatt. Don't try to start a Meltdown which isn't easy to do in Unit 1 but easy in U2, because you'll get kicked (and everyone else) if the Meltdown starts. It's not like a explosion, the rods just melt.