You DON'T.
The audio jacks on the computer DO NOT connect to speaker wires.
The audio jacks connect to the input of AN AMPLIFIER.
the speakers connect to the output of THE AMPLIFIER.
apparently you can convert 3.5mm to speaker wire. however i do agree it is not the best option you have.
soundcards are most likely capable of powering small speakers via the 3.5mm L/R channel but anything more than a very small driver is most likely not going to give you any output without an amp. there are small amps like the fiio which could boost the power on a single 3.5mm but this is normally used to drive high end headsets over 3.5mm not speakers.
an amp with a a digital signal from your pc and actual binding posts for the speaker wire is the best solution and is what you should go for if you want a good solution that is known to work without issue.