You dont.
You need a home theater receiver that can seperate out the bass to send to the sub via LFE cable.
While your PC has the ability for 5.1 or 7.1 surround sound, what it does not have is setting for 2.1 audio.
If you leave audio settings in 2.0 then you wont get any sound going to subwoofer.
If you try to put them in 5.1 or 7.1 then on any source that has surround sound then you will not get the surround and center speaker sounds out of your speakers (which would be very very noticeable in games and movies).