Instructions
Leave the volcano to get to the world map. Click the "Return Home" button at the bottom right corner of your screen.
Click on the backpack icon at the bottom right corner of the screen. This icon represents your inventory. Click the arrows to go through your inventory until you find the fireball item.
Click the fireball icon. Move your mouse over a tile on your home island. Look at the border of the tile when you hover over it. If the border is green, you can place the fireball there. If the border is red, you must place the fireball elsewhere on your island.