Notes on this puzzle
39 digits are printed in this grid and the other 42 cells are yours to fill. A solver that never guesses gets through it in 42 steps.
Nothing beyond hidden single is needed anywhere, so the whole grid is a scanning exercise.
Among the givens, 4 is the most common digit (6 times) and 9 the rarest (2). The emptiest box is the top-middle one, with 3 givens; the top-left box is the most crowded, with 5.
Where to beginLook at row 1, column 6 first: a hidden single is available there from the very start. Scan digit by digit; notes are hardly needed here.