Pick one digit
Choose a number and scan every 3×3 box. Existing copies of that number eliminate rows and columns, often leaving a single valid cell in another box.
Repeat for digits one through nine. This cross-hatching method is especially effective at the start of an easy or medium puzzle.
Look for nearly complete units
A row with two gaps is more promising than one with seven. Compare the missing digits with the intersecting columns to place one or both.
Do the same for columns and boxes. Rotate between them instead of becoming stuck in one area.
Name the reason before placing a digit
A placement should be forced by the current grid. Say the unit and the exclusion: ‘Seven cannot use these two cells because their columns already contain seven, so the remaining cell in this box must be seven.’ If the explanation depends on what might happen later, it is not yet a placement.
Use small candidate notes only after repeated scanning stops producing progress. A candidate means a digit is still possible, not that it is a promising guess.
Rescan the three affected units
Every confirmed digit changes one row, one column and one box. Inspect those three units immediately before moving to a distant part of the grid. The new placement may reduce another cell to one possibility or leave a digit with one legal position.
This local rescan creates a reliable chain of deductions. It also reduces stale candidate notes because each affected mark is checked as soon as the grid changes.
Use a recovery routine when stuck
Return to digits that appear most often in the grid, then inspect units with two or three gaps. Check that no entered digit duplicates another in its row, column or box. If notes are present, remove any candidate contradicted by a confirmed placement.
Do not fill a cell merely to test what happens. On Fun IQ, use Check progress after a short logical sequence. If an error appears, revisit the first unsupported placement rather than replacing several numbers at random.