In order to make a fraction a ratio, you must simplify it if it is needed. So for 12/9, you can divide 3 from the numerator and denominator. That would equal 4/3. Now that it is simplified, you can make it into a ratio. (can be using ":" or using the word "to")
One way it can be used is --> 4:3
Another way --> 4 to 3
Good luck!