Yes, do cut your larger square first but make sure before cutting that you have enought to cut your smaller squares. Use your 15" to cut your 12.5" square as Jan described in her post. You might want to buy Omnigrid Glow-Line 1/4" Transparent fluorescent tape to mark the 12.5" line on your 15" ruler so you don't make any mistakes.

http://www.ebay.com/sch/i.html?_odkw...+tape&_sacat=0

I always mark my cutting line on the ruler/rulers with painters tape. I is too easy for me to get the wrong line. Sometimes I put a small piece diagonally across the markings on the other part of the ruler. That means I don't want to use those lines. This just makes it easier for me NOT to mess up.