I had the same horrible porblem with 100% cotton totes from SanMar. I was using 375 degrees 15 seconds and medium pressure and the bag not only scorched but the design looked blurry. After getting some advice from the Heat Press Forum I reduced the temperature to 350, used the Teflon sheet while preheating for 12 seconds, laid the design covered with Teflon and cooked for 20 seconds. The bags came out just fine. I also put a stack of mouse pads down so the straps and corners of the totes were out of the way. Just as a beginning 'loosen up lighten up and go easy'.
Susan