Park your Nissan. Set the parking brakes. Allow your Nissan engine to cool down.
Open the hood of your Nissan. Look at the frame or under the hood for the serpentine belt routing diagram. It is often printed on a sticker found on the fan shroud.
Locate the serpentine belt tensioner on your Nissan. You will need to move the tensioner away to release pressure from the belt. Take a few pictures of the old belt before removal if you need to look at the belt routing later.
Release Tensioner. Use a 3/8" ratchet to push the tensioner away(up) from the belt. To get to the tensioner, you will either remove the passenger wheel or drive the car up on rams. This will allow you to remove the plastic cover on the wheel well to access the tensioner.
Remove the belt from the accessory pulleys by hand. The belt will need to come off several pulleys including, an A/C pulley, alternator pulley, and crankshaft pulley.
Install the new belt on your Nissan by making sure it is routed as per the belt routing diagram.
The last step is to slide the belt under the tensioner. Before you do that, make sure to check that the belt is properly seated on all the pulleys, or you will have a hard time sliding the new belt over the tensioner.