Gently but firmly start pulling the thread out with your fingers.
Keep pulling until the thread comes out completely. If the thread you are pulling out snaps, find it and start pulling it out again. Once it's out, you will see a "ladder" like line on the fabric. That's your cutting line.
Cut with the scissors on that line. Take your time and make sure that you keep the fabric flat while cutting. You can also use a rotary cutter.
And here you have a perfectly straight cut. Nice and easy.
This method also works on medium weight cotton in solid colours. Have a fantastic day! Larisa xox