This is the most important step. Use the small square of sandpaper in your kit to roughen the area around the hole. This removes the slippery molding agents from the factory and gives the cement a textured surface to grip.
Not every tube can be saved. Replace the tube instead of patching it if: The hole is on a seam. The damage is at the base of the valve stem. Bicycle Inner Tube Patches
Peel the foil backing off the patch (try not to touch the sticky side) and press it firmly over the hole. Use your thumb to apply heavy pressure from the center outward to work out any air bubbles. This is the most important step