Shoe labels are adhesive and should be applied by pressing firmly with your fingers inside the shoe, on the insole. For maximum adhesion (so that it can 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 not porous. Although shoe labels are waterproof, on porous surfaces moisture can enter from below and they can lose adhesion.