OK, so this is really easy.
This is pretty easy! Just peel the backing from one side of the decal / sticker, align with the door, window or whatever you want to install the decal on and adhere the edge and gradually work your way across the decal. Using a soft cloth to make sure no air is trapped. If you get an air bubble, try to work the bubble to the edge or if that doesn’t work, use a pin to allow the air to escape and press down the rest of the decal.