That Number Square
First, we looked at the numbers we had to work with. We had numbers 0 - 99. After that, we sorted them into groups. Group one consisted of the outline numbers, and group two consisted of the middle numbers.
Outline numbers are the numbers at the top, sides and bottom. On the left hand side, the numbers go down in 10s, starting from 0; on the right hand side the numbers go down in 10s, starting from 9. For example, 9, 19, 29 and so on. On the top are consecutive numbers; 0 - 9, left to right. Finally, on the bottom, are consecutive numbers, 90 - 99 and left to right.
We placed all of the outline numbers on the outside of the grid so then we had an idea where the middle numbers needed to be placed.
We placed the middle numbers by working from the outside numbers; hence why we did the outside numbers first. For example, take the number 60. Then you can work your way into the middle by counting to your right.