Shoe Labels are adhesive and must be applied by pressing firmly with the fingers inside the shoe, on the insole. For maximum adhesion (to withstand washing and drying) it is important that the shoe is new, or at least clean and dry before application.
It is also important that the surface of the insole is as smooth as possible, such as leather, leatherette or synthetics, and non-porous. Although shoe labels are waterproof, on porous surfaces moisture can enter from below and may lose adhesion.