When you open the tool, you'll see 1 green dot and 1 red dot. To fill the 10-frame, simply drag the dots to the desired squares. If you make a mistake, just drag the dot back or out of the ten frame area. To start over, click the "Reset" button.
In the Settings menu, you can select the Frame Size: 5, 10, or 20 squares. You can also change the type of counter from dots to fruit: red apples and green pears.