(Dubuque, Iowa, USA)
I am not 100% sure this is the way to go, but look at the square. Is there a 1? If there is not, then look at each row for ones.
The rows that do have ones are not usable. When you do find a row without a one, look at the column. It also should not have a one, letting you know where to put the number.
Do 2, 3, 4, 5… until the square is solved. Hope this helped.