Sometimes, the stereo removal requires special tools which can be had only by purchasing them through your dealer. Be careful and take your time. But also be aware that sometimes removing your factory stereo may affect some other functionality in your car as in the mid 90's Honda Civic, which can disable some functionality in its security system. Best case scenario is to purchase the service manual for your car so you can tell how to remove what you need to. Place panels you remove from your car on a soft cloth away from your vehicle.
Test lights can sometimes blow delicate electronics.
If you are inexperienced, always use the voltmeter. This wire must give ground all the time. A chassis ground is fine. DON'T use this wire. Figuring out the speaker wire's polarity is also important.