Click the menus to choose a flower or defensive unit.  Move the mouse to select a location—the blue highlight shows where units can be placed.  Click again to place the unit.  Plant flowers to attract more lumins.  Spend lumins to buy new defensive units.  Click existing defensive units to buy upgrades.  Your defenses will attack and destroy the invading Muck Beetles.  Experiment with different units and locations to maximize your defenses.