Make useful notes
Notes help you remember; they are not final moves.
Write possible numbers
Choose an empty square and turn on Notes. Enter numbers still possible after checking its row, column and box.
Check them again
When a nearby square receives a number, review your notes. Tap a noted number again to remove it.
Turn a note into a move
If only one possibility remains, turn Notes off and enter the number. Undo lets you take the move back.