What happens next?

How to install a car stereo
Set the parking brake and disconnect the negative cable from your car battery.
Be sure to do this to avoid short-circuiting the electrical system during the installation.
For instructions on disconnecting the battery, see how to disconnect a car battery.
Unscrew any screws that are securing the trim in place. Be careful to remove all screws before trying to pry off the trim or you may break it.