The soldering iron basically gets really hot firstly (give it a few minutes to heat up). You place the soldering iron, the core, and whatever you want soldered together (probably the trickiest of all), the core liquifies from the heat, and covers whatever joint/connection you wanted soldered together, and the core acts as a conductor.
You should probably get some practice first, to get the idea of the behaviour of melting solder, how it clings to stuff, how it'll dry, etc.