Key steps:
1) Prepare a 1024x1024 png or jpeg file;
2) Download fasticns utility to convert the icon to a icns file.(Which was not ready since apple requires 512x512@2x after the retina screen release to the market.);
3) iconutil --convert iconset icon.icns, you will get an icon.iconset folder;
4) iconutil --convert icns icon.iconset, you now get the final one.

<1>.将icns图片转换为png格式(比如将当前目录下mac.icns，这样会生成对应的mac.iconset文件夹)
终端命令： iconutil --convert iconset mac.icns
mac.iconset
icon_256x256@2x.png
icon_512x512.png
icon_512x512@2x.png
icon_16x16.png
icon_16x16@2x.png
icon_32x32.png
icon_32x32@2x.png
icon_128x128.png
icon_128x128@2x.png
icon_256x256.png

<2>.将png图片转换为icns格式(比如将mac.iconset文件夹转为mac.icns，其中2x为双倍尺寸，例如icon_16x16@2x.png尺寸为32x32)
终端命令：iconutil --convert icns mac.iconset

mac制作icns图标