Here is a tutorial:
Open cook window, select your mod on left, put your orange level file name in the “Maps” box, select a preview image, make a title, small description (W/O any special characters)(you can do a full descript once it’s on steam), set the visibility to “Hidden, friends only or public” then add something to change notes and click upload.
You MUST have an image and Title, Decription & change notes MUST have something in them or it will fail.