Mods can be uploaded to Steam Workshop via ingame menu.
Steam workshop mod upload guide by Hracicka:
https://steamcommunity.com/sharedfiles/filedetails/?id=1618510587
NOTE 1: Remember to include preview.jpg in the root of your mod folder. This picture is then shown in SW.
NOTE 2: SW mods can be donwloaded as local from ingame menu. Mods will be then unpacked in your gamefolder/mods/