Most people just peel and place the stickers on their keyboard and experience years of worry free use. But if you think your key tops may have gotten dirty or oily over the years here are a couple of tips.
It's best to turn off the computer before you install the keyboard stickers to avoid characters being typed as you install the stickers. And for best results you can clean your keys with a mild household cleaner or denatured alcohol to remove any dirt or oils. Allow the keys to dry before installing the stickers.
An easy way to install the stickers is to use tweezers or a small knife blade inserted under the corner of the sticker to lift it from the sheet. Carefully place the sticker on the correct keyboard key and when you have the sticker properly aligned, press it down on the key while removing the tweezers or knife blade. Then use a rounded object such as the rounded end of a plastic pen or pop sickle stick (after you've enjoyed the pop sickle of course!) to rub the sticker firmly in place.
This will firmly affix the sticker to the key for years of use!