I've had several people ask me about cutting with a rotary cutter. Instead of trying to explain, which would be painful for both you and me, I thought video tutorials would be a better option! It's so much easier to understand if you can see it in motion.
Here are a few examples of cutting with a rotary cutter that I found on YouTube. There are many different techniques, use the one that seems to work best and is most comfortable for you.
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Happy stitching!