Using your mouse, place the colored block from the fly on the top left of the screen in the grid. Match 4 or more blocks of the same color and make the weapons in the board fall to the bottom.
Once they get to the bottom you can use them to fight off the Flies on the right side. Simply click on them to shoot.
There are blocks with special weapons such as the fire-thrower and the atomic bomb. To use them simply click on the button of the corresponding weapon beside the ammo stack in the bottom left corner.