To decorate your forest with decorations you have collected, tap on a knot on one of the branches in your forest - you will then have an icon pop up and say ‘Place’. Pressing place will then take you into where all your decorations are kept, then you can select the decoration of your choice, and press ‘place’. That's how easy it is! If you'd like to swap out an existing decoration, the process is the same. The button will just say “swap” instead of “place”.