You build in your zoo, make a blueprint of it. Afterwards you go into your blueprint area. If you hover over the little icon of the building you created, you can see the steam button. Click this to upload to steam workshop. You need to give it a title, description and a category. Afterwards make sure to set it to public viewing in the workshop, otherwise its just you and your friends who will enjoy your creations. Hope that helped :)

tysm!