After downloading, start your Messages App, tap the A symbol next to where you would normally type to send a text, and open the sticker pack. Tap or drag the sticker to the text field. You can even construct your own sticker by dragging more in the messaging field.
Stickers drawn on paper by drsenorita (twitter.com/drsenoritashop)
and designed digitally by (twitter.com/RageHu)