I think his stereo will have a amplifier built in (Y)
It'd be worth checking the impedance on the sub and the current that your stereo supplies. If the speakers are too resistant, then they won't work properly. If they don't resist enough, you might blow your stereo.
That's just about covered the "how", now more importantly, the "why": Why on earth do you want to connect your sub up to your stereo that is completely not designed for it? A sub is designed to deal with low-end frequencies, yet you're connecting it to the same thing as a normal (all frequencies) speaker? If it sounds better than it did before, I'll eat my, um, keyboard.