multiple file to single file

I'm trying to change a "folder" file to a single file, or .icns file.
I downloaded a set of icons, but the original author saved them as folders, or multiple files. I've already tried to go ahead an convert them via online converter, but whenever I do, it opens the file as if it were another folder, going into it, but not actually selecting it. I've tried adding the .icns to the end of the file name; I've tried selecting the file image, copying it and pasting it to GIMP with a transparent background, saving as a PNG and then uploading it to the converter; I've tried all kinds of things, but I can't seem to turn a supposed file folder into a single icon.

Please help.

Thanks.