Select the number you want to place in an empty cell from the number panel on the right.

To place the number, move the mouse to the desired cell and double click. If the value is not legal the input will be ignored.

To mark the cell with a potential candidate, single click the cell and the currently selected number will be placed as a candidate.

Clicking a cell a second time with the value that is either a candidate or the final value will remove the value from the cell.

The Clear button can be used to clear or reset a cell.