The rule
Fill the 9×9 grid so every row, every column and every 3×3 box contains the digits 1 to 9 exactly once.
1. Cross-hatching (scanning)
Pick a digit, say 5. Look at a 3×3 box that does not have a 5 yet. Every row and column that already contains a 5 elsewhere rules out cells in this box. Often only one cell is left: that is your 5.
2. Naked singles
Look at a single empty cell and list which digits are still possible given its row, column and box. If only one digit remains, place it.
3. Hidden singles
Within one row, column or box, if a digit can only go in one cell, it goes there, even if that cell has other candidates too.
4. Pencil marks (notes)
On medium and hard puzzles, write the small candidate digits in each cell. Notes mode in our Sudoku does exactly this, and removes nothing automatically, so you stay in control.
5. Naked pairs
If two cells in the same row, column or box both contain only the same two candidates, for example {3, 7}, then 3 and 7 must go in those two cells. Remove 3 and 7 from every other cell in that unit.
6. Pointing pairs
If inside a box a candidate only appears in one row (or one column), then that digit must be in that row within the box. You can remove it from the rest of that row outside the box.
Practice on Easy until scanning feels automatic, then move to Medium. The Daily Sudoku is the same puzzle for everyone, so you can compare times with friends.