Simply click or tap on any Sticker shown to the right the photo (under the Sticker tab) to apply that Sticker to your Photo.
You may also drag and drop a Sticker onto your photo.
Be sure to scroll down to see all of the Holiday Stickers that you can use to personalize your Photo.
Move, scale and or rotate the Stickers to place them exactly as you wish. There is no limit to the number of stickers you can apply to your Photos.