6×6 Sudoku Rules: How Mini Sudoku Works

The three rules

Fill the grid with the numbers 1 through 6. Each number appears once in every row, once in every column and once in each outlined 2×3 box.

The clues already on the board cannot change. Every other number follows from those three constraints.

Find the most constrained cell

Start in a row, column or box that already contains many numbers. List what is missing, then remove any number already present in the crossing column or row.