String pieced block before squaring. (Note I use triangle pieces on the ends of my block to avoid having tiny little corners after trimming.)

I flip block over and trim from my foundation side.

Lining my square ruler up with my diagonal line centered between the two stitching lines of my center string, leaving approximately 1/4 inch of my muslin foundation on the right side and top to be trimmed.

I check my top corner…..

and my bottom corner before cutting. I check here to make sure that my 9 and 1/2 inch mark is slightly INSIDE my muslin foundation (I still have to trim those two sides)

Once I check, I trim off the right side and top.

Flip my block around so the just trimmed sides are on the bottom and left.

Line up my corner with my 9 and 1/2 inch mark, making sure my diagonal line is still fairly straight.

Trim off the excess on the right side and top.

And here’s my 9 and 1/2 inch squared block.