Get a sub that will take the L/R RCA outputs and provide L/R high-passed outputs for the input to your amp. There are many.
What you will find is there are many ways to connect a sub (or a pair of subs). My perfered way is different than Kal's.
I prefer to use subs with speaker level connections - you connect the subs to the same binding posts on the back of your amps (or even the binding posts on the back of the speakers). Just double up the connections.
I prefer subs with variable crossovers that give you crossover frequency options of 30 Hz at a minimum. You'll probably get advice to cross your subs over around 80 Hz. IMHO, that's the best way to hate the sound of subs. For a starting point, multiply that -3 dB down point on your speakers by 0.70 to establish your base crossover frequency of your subs. As always, YMMV.
Also, I prefer subs that give you 0-degrees, 90-degrees and 180-degrees of phase control. IMHO, 2 subs are better than one.