If you've ever cut burlap, you know how hard it is to cut it in a straight line. If you haven't seen this tip before, you'll thank me later. :)
Step 1: Measure the width you want to cut out and use a safety pin to pull out a bit of string at that width.
Step 2: Pull the string out slowly so it doesn't break. If it does, no worries, just find the string and keep going.
You're welcome! :)