The aim of classic Sudoku is to fill the empty cells with the numbers 1 to 9. Each number must appear exactly once in every row, every column and every 3×3 box. The numbers given at the start remain in place.
Before placing a number, check its row, column and box. If that number already appears in any of those three areas, it cannot go in that cell.
For example, a row contains 1, 2, 3, 4, 5, 6, 7 and 8, with one cell left empty. That cell must contain 9. This is a simple example of finding a single candidate.
If a cell has several possibilities, look elsewhere. You can also search for the only position a particular number can occupy inside a box: the filled rows and columns may rule out all but one cell.
Start with a puzzle where you can find a few clear moves. Then open Sudoku and apply the row, column and box rule to a real grid.
Try it in the game
Sudoku