Guide
Three rules, a worked example, and the techniques that solve every board without guessing.
Shade cells so that no number appears more than once among the unshaded cells of any row or column — duplicates are fine among shaded cells.
Shaded cells may never touch horizontally or vertically. Diagonal contact is allowed.
All unshaded cells must remain connected as a single region — a shade that walls off part of the board is illegal.
The board is solved when all three rules hold at once. Every puzzle has exactly one solution, and you can always reach it by logic alone.