(symmetrical, camera angle may make them appear not)
practice and observation
i use the triangle file, flat file, and round file, first i roughly trim away what i dont want with the flat edge files, this is often done squarely, then i trace the contour out like a semi circle and use the round file to achieve the the correct interior contour around the crown...hmm hard to explain in words. then i use sand paper to finely make them symmetrical and smooth and to eventually finish them before a deep polish... my advice is look at a lot of pictures from many angles , hold the gen in your hands if you can and observe it in 3D space. when you go to mod your piece visualize what you want removed from the stock CGs and maintain that image in your head until youre satisfied that it matches up. i find drawing cutaway spots on the actual case to be ineffective