I'm guessing you don't have a 6.5" ruler.
If your block is 7" square, you need to cut 1/2" off in each direction. To do this evenly, since your borders are already even, just cut 1/4" off all 4 sides.
If your block is not already square, but your center is 5" square, then you need to make each border strip 3/4" wide. (5+3/4+3/4=6 1/2"). Lay your ruler so that the 3/4" mark is against the inner side of the border and cut along the edge of the ruler. Do this on all 4 sides.
If your center is not perfectly square, then you have to decide whether it would look okay with the borders uneven, since that is the only way to square up the block without taking off one or all of the borders.