The best way is to put the block under the 12 1/2 inch square ruler and trim any excess off.
Another way is to use the grid on your cutting matt. Be careful and don't do this one until we talk about it in class.
The last method, and not one I highly recommend is to measure the width at at least four different spots
The height at four different spots. and then do a visual check to see which small bits may need trimmed off. 
