1. In the main menu Click Create workshop item

https://steamuserimages-a.akamaihd.net/ugc/955212773165774825/72880C2C21E4A713A6A1CF6FCCF1CC6134DDD584/

2. Type in name and description and Click Upload

https://steamuserimages-a.akamaihd.net/ugc/955212773165774997/E17C7876D6B7496F36E4B0AAF9211E08851C9C3F/

3. Take a note where your local folder for the item is, for example this would be

.....\steamapps\workshop\content\890180\1463068810

Wait few minutes as Steam needs to process your request and make it visible, this usually can take up to 15 minutes!

4. Upload your artwork to that folder, in this folder you can also find the icon, which will have the name that you gave to your item. If you replace and it with your own, it will be uploaded and used in workshop.

5. In the main menu click Upload workshop Item

6. Choose your created workshop item from the list

https://steamuserimages-a.akamaihd.net/ugc/955212773165775122/C8C1A68FB253F9F175B13CAEE719D051FFA48C2F/

7. Click upload

Wait few minutes, so the Steam can digest it :)