First you are want to think of what two numbers will give you 72. It gives you 6, so you need to think of 6 times what will give you 72 (which is 12). Next you want to break down the 12, and you do the exact same thing you did with the 72. It gives you the number 3 so you need to find out 3 times what equals 12 (which is 4). Then you break down the 4 the same way. You have to do that to the 6 as well.
You continue to break down numbers until you can’t anymore.