Simply load your created environment mod. Do some changes if needed, you can also update them later, when the mod is uploaded to the workshop.

Press the save button. The menu of your creations will pop up.

Click on the + button


It will open a menu, where you will set the name, choose the category and thumbnail picture.

When you are ready, click on the Upload button located down at the right.


And that's it, congratulations. You can find your uploaded creation in your steam workshop tab.