When you purchase a decoration in June's Journey, you can place it on any open area of 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.