in my malibu, I loosened the belt tensioner first. There is a motor mount just in front of the belt that ou have to take one side off of. I lut a jack under the engine and jacked up the one side about an inch and slid the belt between the mount and the frame. Replacement belt should be put on using the diagram shown under the hood. It took me about 30 minutes my 1st time.
I'm not going to dispute anyone else's answer, but if you want to know a real easy crude way to change the serpentine belt on a 99' Chevy Malibu in about 5 minutes, which I just got through doing 15 minutes ago, listen to this.
You do not need any tools other than a long piece of steel bailing wire and may-be one other person to take the belt off the pulley. Run the bailing wire around the belt tensioner and out to the front of the car, pull the wire towards you and there you have it, bingo, the belt is loose enough to come off with out removing any other parts from the car. In order to replace the belt, just do the same thing, pull on the wire and install the new belt. (DON'T FORGET TO REMOVE THE WIRE AFTERWARDS) lol