Hit the Clear button and enter the Sudoku 'givens' manually. You can do this by
clicking on a cell, or use the keyboard arrow keys, then type the digit from '1' to '9'.
The Delete key or the '0' key will clear a cell. You can also enter a digit or clear it by clicking in the row
of digits above the board.
When you are finished hit the Load button. The givens will now appear in blue.
Hitting the Reset button will now undo all changes and take you back to the original givens.
The allowed digits are shown as a small light gray digit in the background of each cell. You can turn this off by unticking the 'Show allowed' option.
If an allowed digit is the only possible digit (these are called singles) then it is highlighted in red.
You can turn this off by unticking the 'Show singles' option.
Hitting the Accept button will convert singles to actual entered digits.
The Hint button will fill in the current selected cell with the correct digit.
Clicking in a cell with a digit entered will highlight the same digit anywhere alse in red.
Solve of course will solve the entire puzzle.
Undo will undo the last user action such as entering a digit or htting the Accept button.