I saw a cool trick the other day and tried it for my chicken blocks....
take a piece of paper and cut out carefully the size of square you want to use. Keep BOTH pieces, the square and the bit with the square shaped hole. ;-)
Use the piece with the hole in it to "audition" your fabric, then when you're happy with it, put the "square" that you cut out back into the hole. Remove the paper with the hole leaving the square in place, and use a rotary cutter and ruler to cut around it.
Did that make sense?