Pads can be placed in several areas and still remain effective. For people who are self-conscious or want to be more discreet, put the pad between the fitted sheet and the mattress pad. The drawback here is that the fitted sheet will get wet which could make the person uncomfortable.
Another most common place to put bed pads is to put it on top of the fitted sheet so that the bed linens stay (mostly) dry and there is no moisture against the person’s skin or bed clothes.
Of course, make sure they are right side up and the absorbent layer is towards the user and the waterproof backing is against the bed. The longer side of the bed pad should go across the bed, not head to toe.