To block the squares, I first pin one corner to the mat:
Then, I take my awesome clear ruler and measure downward to get my desired dimension:
The important thing to remember is that you'll want to pin the squares at the point of the miter, so they're nice and even. The tension pulling on those mitered corners will keep the whole square from deforming.
After I get all 4 corners pinned, I smooth out the square and pin along the sides evenly:
As I say in my video below, I use two mats to protect my table tops. This helps the pins keep a good grip on the square without creating large holes in the mats. It also protects my hands if I have to move the mats around while they're still blocked:
Here's the aforementioned video/ review. Hope it helps!
PS- You didn't think I'd forget the giveaway mention, did you?