What is the correct way of converting one or more .png files to .icns files?.
icnsutils but I'm having a hard time figuring out how to work with
You can use:
The first one is the name you want your .icns file to have. The last one is the .png file you want to export as an icon.
Important: Only convert the icons that match sizes icns supports: (16x16, 32x32, 128x128, 256x256, 512x512 and 1024x1024 pixels)
Or you can use an online service: iConvert Icons
Source: Ubuntu manuals