once the cake is iced completely you can start sticking leave to it. i randomly placed leaves where ever i wanted, though i did try to spread it out so that the orange leaves were evenly spread amongst the brown.
after you feel you have completely covered the cake, take you acorns and apply a small amount of icing on the side you decide is the bottom. again the icing is serving as your edible "glue", now decide where you want the acorns and place them there, icing side down.
i would suggest only placing acorns on the top of your cake, while the icing is a "glue". acorns stuck to the 