Fill the grid so every row contains the digits 1 through 9 exactly once
Fill the grid so every column contains the digits 1 through 9 exactly once
Fill each 3×3 box so it contains the digits 1 through 9 exactly once
Start by finding rows, columns, or boxes with the most filled-in numbers
Look for numbers that can only fit in one spot in a row, column, or box
Eliminate impossible numbers by checking the related row, column, and box
Use pencil marks to note possible numbers in empty cells
Recheck cells after every new number is placed
Repeat scanning rows, columns, and boxes until no more moves are obvious
Use more advanced logic only if needed
Continue until every cell is filled correctly