Scanning gets you through easy grids. The moment it stops working, the next tool is pencil marks — writing every candidate into every empty cell so the patterns become visible instead of imagined.
Why notes beat memory
Holding six cells' worth of candidates in your head is possible for about a minute. Written down, the same information sits still long enough for you to notice that two cells share exactly the same pair of options.
Naked pairs, the first real pattern
If two cells in the same row, column, or box have identical two-candidate lists, those two digits are locked into those two cells. You do not know which goes where — and you do not need to. Every other cell in that unit can drop both digits.
Hidden pairs are the same idea, inverted
Sometimes a pair hides among longer candidate lists: two digits that can only appear in two cells of a unit. Those cells belong to that pair, and every other candidate in them can go.
Keeping notes clean
Stale notes are worse than no notes, because they lie with confidence. This site strips a digit from every affected cell the moment you place it, so what you see is always current.