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ROSA Sudoku
A useful starting point in Sudoku is the intersection of three constraints: the row, column and 3×3 box. Each of them removes possibilities from an empty cell.
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Find a single candidate
Choose a cell and rule out digits already present in its row, column and box. If only one candidate remains, you have a logical placement. Check nearby cells afterwards: a new digit changes the possibilities in several places at once.
Track one digit across a box
A cell may have several candidates while a particular digit has only one possible location within its box. Follow the rows and columns that exclude the other positions. This finds another kind of forced move without trying random answers.
Give each placement a reason
Before entering a digit, ask which constraint proves it. If you cannot explain the answer, examine another part of the grid. Undo can correct an entry, but it cannot replace that check. Start with a comfortable difficulty and move to harder puzzles gradually.