When you open the tool, you'll see one green dot and one red dot. To fill the ten 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, tap or 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 fruits: red apples and green pears.