To fill the squares look for the one filled in column/row/diagonal and figure out its sum.
All other columns/rows/diagonals must be equal to that, so whenever there is a column/row/diagonal with a single missing value you can add the others up and calculate the missing value.
So you have to find those single missing value columns/rows/diagonals, fill them out and continue with the next one until the whole square is filled in.