Some folks are really good at judging where to cut using a ruler and their fabric. I don’t happen to be in that category. I need to use my mat and some exactness. So on the cutting of the 9 patch blocks I ironed my lines using the middle block as my guide. I lined up the seamed edges (top to bottom), folded the block and ironed.
Then you fold the block in half by lining up the stitches of the center square. You should then have it perfectly centered. Iron the block, unfold and you’re ready to cut.
I’m even fussier when it comes to lining up the guide. The good thing about a 9 patch is that you have your seams to line up with the cutting guide, and your cutting mat as a guide. I put the center onto one of the inch marks, then straighten it on a perpendicular line. The guide has a lot of rulered lines on it and you can line that up to a seam to make sure you’re square with the fabric. So very little room for mistakes on your blocks.