Ideally you make a Low Pass Filter and Drive another Seperate Power Amplifier for it. This amp should Also be Higher Power as BASS Requires LOTs of Power.
It is also should also have a Mixer before the Low Pass Filter so it Contains the sounds from Both Left and Right Channels.
Never going to be very easy! - for a start you need a special dual voice coil speaker, and passive crossovers. This is likely to cost MORE than a conventional speaker and amplifier - and not perform as well.
As already mentioned, you need more power for the sub-woofer, so by doing it passively you're drastically reducing the power to the other speakers.