Look at an empty cell and list which digits already appear in its row, column and box.
Naked Singles
The simplest deduction in Sudoku — a cell with only one option left.
A naked single is an empty cell that has exactly one remaining candidate, because the other eight digits already appear somewhere in its row, column or box. Whenever a cell is down to one option, you can place it immediately.
Worked example
5
3
7
6
1
9
4
8
2
1
How to apply it
Cross those off the digits 1–9.
If a single digit survives, that is the answer for the cell.
Place it, then re-scan — a naked single often creates more.
When to use it
Naked singles appear constantly, especially late in a solve as the grid fills up. Keeping pencil marks makes them obvious: any cell showing one note is a naked single.