Find the crease line on the side, under the sleeve. It's always been nice and straight for me and I just cut up to the seam on it. I have also used sweatshirts that have a seam down the side that can be cut open.
Cut open the sleeve, either with a rotary cutter, scissors, or by removing the threads.
Than you have a big, floppy, chopped up rag.
The side crease
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Cutting the sleeve seam
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Sweatshirt cut open
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