Remove the old, worn pads by pulling them off the helmet's plastic.
Pour some rubbing alcohol on a rag and rub the helmet's interior to remove any glue residue from the old pads.
Glue the backs of the replacement pads and press them firmly into place where the original pads were located. Use glue that is safe on plastics, such as a 3M product. Regardless of the glue you use, ensure it will not disintegrate the plastic or the pads. Some epoxies, for instance, will "eat" through certain materials.
Let the glue dry the required amount of time as specified on its package. Press the pads firmly into place occasionally to ensure they are sticking to the plastic.