You probably need to contact an audio/auto electronics specialty shop and have them do this for you. This requires tapping into correct wiring and running/finding battery power inputs and ground points. Probably a bit technically steep for you at this time if we're being honest. I agree the "premium" sound system in the 2008 CR-V EX-L isn't much to write home about and that under-seat subwoofer is super weak - I own one myself, and it's lacking seriously.

I have a 10" Rockford Fosgate woofer/amp combo and it absolutely pounds for what it is, but you still need to tap into the subwoofer input wires and hook up the amp power and ground.