Any time you work on your vehicle you should start by putting on the necessary safety equipment.  Eye protection will prevent dust, fluids or debris from falling into your eyes if you have to get under the vehicle.  Gloves can also provide protection against scratches, pinches and cuts that may result in working in your engine bay if you choose to wear them.  Always wear eye protection when working on your vehicle. Gloves can protect your hands, but are optional for this project. Open the hood of your car and locate the battery.  It's important to note that some vehicles keep the battery in the trunk instead of the engine bay for space saving or weight distribution.  Once you've located it, use a hand or socket wrench to loosen the bolt holding the ground cable to the negative terminal on the battery.  The negative terminal can be identified by looking for the negative symbol (-) or the letters NEG.  Loosen the bolt until you can slide the cable off of the terminal.  Tuck the cable to the side of the battery to prevent it from coming in contact with the negative terminal. This will ensure the engine cannot be started while you're working on it, protecting you and the engine from damage. Many newer model vehicles come equipped with an auto-tensioner pulley.  The pulley presses upward as the belt wraps around it so the belt remains under constant, steady tension.  Use a half inch drive breaker bar or socket wrench, and slide the portion of the wrench that would normally go into a socket into the square opening in the center of the pulley.  It is possible that your vehicle uses a 3/8 inch drive or even a bolt on the tensioner pulley, so refer to your particular vehicle's service manual to ensure you have the appropriate tools.  Rotate the wrench clockwise to relieve the pressure it is placing on the belt.    It may take a good amount of pressure to rotate the tensioner pulley down.  Be careful to release it slowly with steady pressure to avoid injuring yourself or damaging the vehicle. As soon as you let the tensioner go, it will return to its usual position and put pressure on the belt again, so you will need to hold it in place as you remove the belt. Not all vehicles are equipped with an auto-tensioner.  Consult your specific vehicle's service manual to see if yours is. If your vehicle does not have an auto-tensioner, you can still relieve the tension on the belt by loosening the bolts attaching the alternator to the engine.  As you loosen the bolts, tilt the alternator forward to create slack in the belt.  Be careful not to damage the threads of the alternator bolts by allowing it to sit tilted under pressure for too long.  If your vehicle has an alternator belt instead of a serpentine belt, this is the method you will need to use. Be careful not to damage the wiring going into the alternator.  The alternator provides the power the engine needs to run through those wires. Once the tension is off the belt, slide it off of the first or highest pulley.  Once it is off of one pulley, you can release the auto-tensioner if that is the method you used to reduce the tension.  Make a note of how the belt is wound throughout the different pulleys if it is a serpentine belt, as you will need to slide the new belt on in the same way.  If you do not have a diagram on the vehicle or in your service manual laying out the direction your serpentine belt travels through the pulleys, take a picture of it with your phone to refer to when installing the new belt. A strictly alternator belt will likely only go around an alternator pulley and the crank pulley at the bottom of the engine.
Put on the appropriate safety gear. Disconnect the battery. Use a breaker bar or socket wrench to loosen the auto-tensioner. Loosen the alternator bolts if there is no auto-tensioner. Slide the belt off of the pulleys and remove it from the car.