When you purchase a Decoration in June's Journey, place it on any open area on Orchid Island.
When you are about to place a Decoration, you will see a green or red colored border around it. If the border is red, that means you cannot place the Decoration in that spot because something is blocking it. If you see a green border, it means you can place the Decoration or Building in that spot.
You can also move a Decoration or Building around after you have placed it by clicking/tapping it, dragging it to a new location, then tapping the green "Check Mark" button to confirm the move.