Go behind you radio and get to the ISO harness.
Splice the front speaker conenctions (see link below, although most ISO looms are labelled up well).
[ame="http://en.wikipedia.org/wiki/Connectors_for_car_audio"]use this link to help[/ame],
Feed the audio out from you headunit into the inputs of your crossovers. Feed the midbass out (or low, LF, bass) back to the cars wiring loom, through the other end of the splice you made on the iso loom.
Run new cables behind the dash, to the A pillars for your tweeters. Drill holes on the A pillars to screw the tweeters in and feed the wire through. To get the tweeter wiring behind the dash you may need to remove the weather strips (rubber) between the dash/cabin and doors. Something should be possible, have an explore!
You'll need a fair amount of speaker cable, some crimps, electrical tape or heatshrink. Decide where you put your crossovers, maybe wire-tied behind the dash, if they're small or under the seat, attached to some wood if they're big.

If you're planning on amping things, under the seat is probably a good plan!