It is hard to describe how to square up blocks, you want to do so without losing points or having lopsided squares.
I try moving the square up ruler around the block, auditioning different places that I "could" make the cuts.
With the D9P, I would probably start by placing the ruler in the center of the block, lining up seam lines with the rulers markings. This ensures that your block won't end up "wonky." I would try to cut the same amount of fabric off, all the way around the block.
If you placed the ruler in one corner and started cutting, two sides would have squares that are smaller than the other two sides.