Draw a line. A single line. How complex can that be?
Unium now features an integrated level editor! Simply click "Level Editor" in the main menu and you'll be able to create new levels and upload them to the Steam Community Workshop! Visit the workshop page to subscribe to any you'd like to try, and re-run the game to download them all.
We've made a handy dandy [url=http://steamcommunity.com/sharedfiles/filedetails/?id=438507864]guide[/url] discussing the process, which hopefully everybody finds easy to use. The big catch is that you'll need to prove you can solve a puzzle before uploading, so everything that finds its way onto the workshop should be solvable.