The sum of the nine numbers from 1 to 9 is 45:

So, the three rows will have sums that add to 45. Also, the three columns will have sums that add to 45. Choose three different numbers (greater than 5 and less than 25) that add to get 45. Give the rows (or columns) these numbers as sums, and then try to place the nine numbers.

For example, 10 + 15 + 20 = 45

Try to put the nine numbers in the grid so the top row has a sum of 10, the middle row has a sum of 15, and the bottom row has a sum of 20.

Then, check the sums on the columns and diagonals. You may need to do some "moving" of the numbers to make the sums all different.