The rule is small; the consequences are not
In standard Sudoku, a digit may appear once in each row, once in each column and once in each three-by-three box. These groups are often called houses. A cell sees its row, column and box, so any digit already present in those houses is excluded.
That is the whole rule set. The rest is bookkeeping, pattern recognition and the occasional moment when a single pencilled digit behaves as though it has solved a philosophical problem.
Scan before making notes
Begin by looking for a house with only one empty cell: its missing digit is forced. Then look for a digit that can fit in only one place within a row, column or box. This is a hidden single. These moves are simple, quick and worth repeating after every placement because one new digit can alter several houses.
Do not rush to fill every empty cell with candidates. On an easier grid, scanning may carry you a long way. Notes become useful once the obvious information has been collected, not as a ceremonial first layer of fog.
Candidate notes should stay legible
For each unresolved cell, list the digits not already seen in its row, column and box. Pencil marks are external memory. Keep them current: remove a candidate when a new placement rules it out. Untended notes make a grid look impressively busy while contributing little except a faint administrative dread.
Some solvers use centre marks for candidates and corner marks for more tentative observations. Any convention works if it lets you distinguish a possible digit from a confirmed answer at a glance.
Pairs and locked candidates
A naked pair occurs when two cells in one house contain exactly the same two candidates. Those digits must occupy those two cells, so they can be removed from the other cells in that house. A hidden pair is similar, except two digits appear only in the same two cells even if those cells hold additional notes.
Locked candidates connect a box with a row or column. If all possible positions for a digit inside one box lie on the same row, that digit cannot appear elsewhere in that row outside the box. The reverse applies when a row confines a digit to one box. These methods do not add information; they make existing information travel farther.
Escalate methods slowly
Harder puzzles can use triples, X-Wing patterns, colouring and more advanced techniques. Learn them one at a time, with diagrams from a reliable Sudoku reference, rather than attempting to memorise an entire alphabet soup. The important question is always the same: which placements would force a contradiction if this candidate were elsewhere?
Guessing is a choice, not a badge of shame
Some digital puzzles invite trial-and-error. If you use it, copy the grid or clearly mark the branch. For a deduction-focused solve, return to scanning and candidates until you can state why a digit belongs. The satisfaction comes from the chain of necessity, not merely from seeing the final grid become tidy.