Brand new serpentine belt keeps coming off?

Nine times out of ten it will be the serpentine belt tensioner you can tell if the belt is not riding in the center of the tensioner pulley. Here is how to change the part out in the images below. Also, once the belt is off, please check the two idler pulleys as shown below. The belt needs to ride in the center of all pulleys. Check out the images (below). Let us know what happens and please upload pictures or videos of the problem.