Use fairly thick cardboard (at least 1/8-inch thick)
Cut a square to the finished size you want
Draw a diagonal line from corner to corner of the cardboard square
Glue sandpaper, fabric, or anything else, to the bottom of the cardboard square so it won't slip
Lay the cardboard square, lined side up, on top of your oversized patch. Line up the diagonal line on the cardboard with the diagonal line in your patch
Cut the patch to size.
I supposed you could do the same thing with something clear, like template plastic. You'd have to glue dots of something to the underside so you could see through to your patch. Just make sure whatever you use is thick enough that you can't easily cut through it by accident.
They make square rulers with grids and lines that'll work, but I'm a bit clumsy and heavy-handed with the rotary cutter. It slips out of place and I'm right back where I started ... every square a different size and out of whack. :cry: