First, thoroughly clean the glass to which the decal is to be applied. Fill a bowl (or flat dish) with clean, lukewarm water. If you intend applying lots of decals, make sure you change the water regularly.
Soak one cut out design at a time until the slide starts to uncurl and flatten out. Do not leave the decal in the water too long, as it will become very difficult to handle.
Remove the slide from the water and gently slide it off the backing paper and into its desired position on the glass. Try not to move the decal around too much, as this will leave a residue or film on the glass.
With your finger on one edge of the decal, gently run the squeeze over the slide to remove any remaining water and air bubbles beneath the slide. If you don't have a squeegee, gently pat the decal surface dry to remove any water and air bubbles.
Allow the slide to dry for four to eight hours (or better still, overnight). Before firing, gently clean the glass with Windex (or a similar product) - paying particular attention to the area around the slide. Be very careful not to touch or wet the slide. The slide is now ready for firing.