Let's see... The big corner triangles:
if you look at the upper left you have two blocks side by side they are 12.5 unfinished, and when you sew those two together they will be 24.5' Cut a Square with sides of 24.5" Now cut it twice diagonally each way. Now you have the four big corner triangles you needed. SIMPLE. :D Now here's a big tip. if your blocks are a little small and your triangle is a little bit big find the center of both, pin from the center out, this will put any extra in your seams.
The little Triangles on the sides:
These are easy. Your blocks are 12.5 unfinished. Therefore the two sides are 12.5. Take one 12.5' SQUARE cut it in half diagonally one time and you will have two of those little triangles. Since you need 6 of these little triangles cut 3 squares.
See no math. :D
I love this pattern it's beautiful!