Fill the grid so that each column, row and block contains only one instance of each symbol.
When the game starts you are prompted to choose which game type and board size you are interested in. You can enter in a puzzle from another source or have KSudoku generate a puzzle. The generator has several difficulty settings and some symmetry settings, which affect the pattern in which the puzzle is generated. Your game experience will depend on which of all these options you choose.
When the puzzle board appears, take a look at the left hand side. There is a selection list which contains all the symbols available in the current game. The set of symbols depends on the size of game you choose. Games may have 4, 9, 16 or 25 symbols. The first two sets use the numbers 1 to 4 or 1 to 9: the others use the letters A to P or A to Y.
First, note the boundaries in the game board. The thick lines outline the blocks. Each of these blocks has to be completed using the symbols available and each symbol must be used once and only once within the block. The same is true of the rows and columns in the board.
Now you can start entering symbols into the vacant squares on the board. To do that, select the symbol you wish to enter from the selection list on the left, then use the mouse to click on a vacant square in the board. Alternatively, you can hover with the mouse over a vacant square or use the cursor keys to move around, then use the keyboard to enter a symbol.
In Roxdoku (three-dimensional) puzzles, always use the mouse to set a small cube to a value and use a double click. When you rotate the large cube, by clicking and dragging with the mouse, it is easier to see what values are on the small cubes in the background if you click between the small cubes, not on them.
It is useful sometimes to enter markers or notes, which are tiny symbols within a square. These have no influence on the solution, but they can help you keep track of ideas you have had, especially when there is a choice between two alternatives and you do not yet have enough information to decide between them. To enter a marker or note, click with the right mouse button or hold down the Shift key when using the keyboard.
To clear symbols from a square, hover over the square with the mouse and hit Delete or Backspace on the keyboard or click with the mouse button on the square.
Notice that the squares change color while you hover the mouse over them. This is to help you keep up with the rules of KSudoku. Use the vertical and horizontal lines to check if the symbol you are about to enter into a vacant spot is already in use anywhere in the vertical or horizontal line that square is a part of. The coloring also highlights the block which, if you remember, must also contain only one instance of each symbol. If you do not need the highlighting, you can use the → menu item to turn it off.
Keep entering symbols in accordance with the rules until the whole game field is filled up. At this point KSudoku will check if all the entries are valid and will either correct you or accept your solution.