Scan the grid for rows, columns, and 3×3 boxes with the most filled cells
Identify missing numbers in each row, column, and box
Eliminate candidates that already appear in the same row, column, or box
Fill any cell that has only one possible number
Fill any row, column, or box that has only one place for a specific number
Repeat the process after each new placement
Use pencil marks for possible numbers in empty cells
Look for pairs, triples, and other candidate patterns to eliminate possibilities
Check for hidden singles in rows, columns, and boxes
Continue narrowing possibilities until the puzzle is complete