Hot glue gun
1. Pluck off some keyboard keys from an old keyboard. The background of my alphabet label stickers was black so black keys were perfect for me. You can spray paint the keys before putting on the stickers if the color doesn't match.
2. Figure out how you want to arrange your keys in the shadow box. Then take some hot glue and simply cover the bottom of the key and then press it into the shadow box. The backdrop of my shadow box was was fabric over some foam so it was very easy.