https://www.upscaleaudio.com/pages/subwoofer-tipsbasically you run a 2nd set of wires from the amp to the sub... however... this is sub (pun?) optimal as there will be no high pass filter on the speakers, and as such they will still be trying to be full range and trying to do the same job as the sub woofer .. you may want to mess with the levels so that the sub and speakers aren't overlapping too much.
Also if you live near other humans be careful with sub woofers as they are a good way to p*ss your neighbors off... seriously if you have shared walls with others.. maybe don't even use a sub... I have a couple subwoofers but I leave them unconnected now that my old inconsiderate neighbor moved.. I previously used mine to demonstrate I could be more of a pain in the ass to him than he could to me.. surprisingly effective..