To share levels you first need to accept mod.io terms of use then need to create a level in the Atelier. You can use any level you have beaten in the campaign as base or start fresh. Once satisfied with your level, you'll need to beat your level to verify it. Once it is done,  you can use the photo icon on top to create a miniature for your level and clic the "sharing button". Give it a name and a short description and you will have shared your level!