Fill in different digits on each row and in each column. Next to the puzzle is stated which digits are valid. At some places there are < and > symbols between neighbouring squares. Then the digit to which the arrow is pointing, is the smallest of the two. As an example we will solve a puzzle with the digits 1 to 3.
The digit in the upperleft corner is smaller than the digit below it. This number itself is smaller than his right neighbour. Because only the digits 1, 2 and 3 are in the game, it must be that the 1 is in in the upperleft square, the 2 below it, and the 3 to the right of the 2.
On the second row and in the left column one digit is missing, but because we know everywhere there must be the digits 1, 2 and 3, it is clear what needs to be filled in.
The top row still needs two digits, the 2 and the 3. It is clear the middle square must be smaller than the 3 below it, so this must be the 2 and the 3 will be its righthand neighbour. Of the two missing digits that remain, we know which one is the largest, so we can complete the puzzle.