Connect the wiring harness to your stock connector. There will be a clip on one side that connects to the clip from your car.

If you do not have the correct adapter for your aftermarket speakers, you are going to want to cut the existing connector off and crimp the new speaker wires to the existing wires in your car, as shown in the photos.

Once these wires are crimped, you can proceed with connecting them to the back of the speaker by sliding them onto the male connectors.

Finally, use self-tapping screws in the holes you drilled in the prior step.