The red arrow is the area that needs to be completely cut off. The green arrow on the wings/tab needs to be filed down thinner on both sides to fit (this step can be skipped if it fits). The top green arrow is another area that can be skipped if the arm (black part) doesn't protrudes above the hop up chamber at all. You can see if it does by turn full hop on, then full hop off. Just file down if it does protrude but don't file too much. It just needs to be flushed with the top of the chamber.