Copy only the digits you trust
Start with the puzzle’s printed digits. If you have already filled some cells, leave uncertain guesses out. A guess can accidentally rule out the intended solution.
Check row by row
Enter digits 1–9 and leave unknown cells blank. After copying each row, compare it with the original before moving on. Check the nine 3×3 boxes as a second pass; this can catch a digit entered one column too far over.
Understand a conflict
A repeated digit in a row, column or box breaks ordinary Sudoku rules. If a conflict appears, inspect the highlighted cells against the original puzzle. Avoid replacing a number solely to make the warning disappear.
Understand multiple solutions
A grid can have no immediate repeated digits and still fail to determine one unique solution. Multiple solutions often mean a given was missed, though some supplied puzzles are genuinely ambiguous. Compare your entered grid with the original before proceeding.
Use a small hint when you want to continue
“Give me one hint” supplies one value from a uniquely solved grid. Try continuing from there. The hint reveals a digit; it does not explain a logical technique. The full solution is available when you want to check the whole puzzle.