The blank board begins with "ticks" showing what numbers are possible in each cell. At the beginning, all ticks appear in every cell because all numbers are possible to start with.
To set a number in a cell, left-click on one of the ticked numbers in the cell.
To undo setting a number in a cell, right-click on the number.
As you put numbers in the cells, the ticks in each of the other cells automatically update to show what numbers can possibly be placed in the cells.
You can manually change the ticked numbers. Right-click on a ticked number to clear it and right-click again (on a tick that was previously cleared manually) to restore it.
Note, if you clear a previously numbered cell, by right-clicking on the number, it will reset all the manually cleared ticks.
To save a board, left-click on "Saved boards:" at the bottom of the board. To restore the last saved board, right-click on "Saved boards:".
Saving the intial board before you start the solution will allow you to go back and start again if you reach a dead-end.