When you begin a new sudoku, some of the squares will have numbers already in them, and have a gray background. The easier the sudoku, the more squares like this you will have. These squares cannot be altered
To place numbers in the Main Puzzle Board,
First select a number from the 'Current Number Selector' by clicking on the number you want. The chosen number will then have a gray background. Then click on the square you want in the Main Puzzle Board. The number will appear in the square.
To remove a number from the Main Puzzle Board,
Select the blank square from the 'Current Number Selector' and then click on the number you want to remove in the Main Puzzle Board. THe square will go blank. Alternatively, If you have just placed a number in the main puzzle board, and have the same number selected in the 'Current Number Selector' then just click on the Main Puzzle Board and the number will be removed.
Once you have filled up all the squares, the program will tell you that you have completed the puzzle. If you fill in the last square and nothing happens - Bad Luck! One or more of the numbers is incorrect, and the puzzle is not correct. At this stage, you will probably be tempted to try the 'Check Moves' or 'Show Solution' button - but don't give up!