Instructions are in the README.txt in the game folder.

Thanks for your reply, but the readme file just shows you where to put the files, not how to acquire them.

I wan't to know if there is a sneaky method of downloading the Steam Workshop maps for the non-Steam version. But it doesn't appear to be possible.

Oh i see.
Well i think you have to wait until there is some kind of community website that allows to upload and index them.
Maybe the game authors will/should do that.

Or create your own maps :p

Wasn't aware there were community made levels available on the Steam version until seeing this thread. There's also no manual method of downloading Workshop creations without owning the game on Steam. Disappointing indeed.