Sudoku rules are very simple, hence its popularity. You have a board with 81 cells, distributed in a 9x9 grid. You are supposed figure out which digit, one to nine, goes in which cell. There's only one possibility. Some cells are filled - not just to make your life easier, but to ensure uniqueness. You cannot repeat a digit in a row, column and in a 3x3 box. That's a sudoku identity, nine rows, nine columns and nine boxes.
The setter ensures that there's only one possible solution for the game. It's your job to find that out.
You may want to start with the most populated column, row or box. That way, you'll have just a few digits to try, or who knows, just the one you need. Eliminate all of these and will probably make good progress. With time, you'll come up with you own technique, but I doubt it was never tried.
Select a number in the tray below the grid. Then click on an editable cell. That number will go there, and whatever was there will be replace. Or deleted if that digit was already there.
The pencil button enables pencil marks. When you select a number and click on a cell, if one of the pencil marks are enabled, you will place numbers in the corner of the cell, or in the centers. These marks are just aids and will not count as a score.
When the eraser tool is activated, you guess, the cell you click will be cleared.
The timer will start automatically when you click anywhere. When you have a complete grid, click the check result button to check if you completed the board successfully.
you may save your progress to continue in another occasion. At the moment you can only save one game at a time. Therefore, you will erase existing games if you save a new one.
If you wish to continue where you left off, use the load button to load the last saved game.