I am currently working on the finishing directions for the Jingle BOM and I have a little tutorial here about trimming large blocks. I'm using my vintage re-make to demo here. This block finishes at 24" and needs to be cut to 24 1/2". When we do the Miss Kringle center, that will need to be trimmed to 27 1/2" but the technique is the same.
First I mark a 24 1/2" square on my dining room table using masking tape. The tape is 1/2" wide and it is 24" from inside edge to inside edge. I also put a couple of pieces of tape through the center so I could center the block.
Next I put the block on this marked square and center it.
From here I mark along the tape (inside edge) to draw the 24 1/2" square onto the quilt block. After that looks good and centered, I trim the block with scissors.