You will notice when you are playing the game that there is a small Pencil Icon In the top right corner. Pressing this will pause the game, bringing up all the functions you can use in the game.
On the top row, you will see a Blue Sticker with a Star icon. Simply open this menu and a list of stickers you can use will appear.
Choose any of these and you will then get the option to adjust the size of the sticker. Click the button in the bottom right corner and the game will ask whether or not you want to use the sticker. Select 'Yes' and the trophy will unlock.