You need a crossover network, the subwoofer speakers (s) and a box for a passive one.
If your amplifier has a line level subwoofer out you can dispense with the crossover network and get another amplifier to drive the subwoofer(s) and a box.
The best option would be to try to buy a pair of speakers, building or trying to design it, its more expensive and time wasting, but my fane boxes play nice