• SteamCMD
  https://developer.valvesoftware.com/wiki/SteamCMD
  
• A mod ready to upload/update.
  
• A text editor such as Notepad++, VIM, VS Code, etc

Store all the content for your mod in one folder. This would be UTX, U, INT files etc.
Make sure only content which is part of your mod is in this directory.

In the directory of steamcmd.exe create a text file in your text editor of choice.
Save the file with the extension ".VDF" (Name it as you wish). This will be your mod config file.

Follow the format below. Make sure to replace examples with your own stuff.

"appid" game id, you don't need to worry about.
"publishedfileid" set to "0" will create a new workshop item. Change 0 to your already existing mod id if you want to update it! (You can check your mod id by copying workshop page link of your mod, the id you need is the set of numberes at the end of it)
"contentfolder" is where your maps, textures, scripts, and other folders go.
"previewfile" is the image for your mod. It is reccomended for it to be square (like your Steam avatar), it should be under 1 mb, and it should be a .png or a .jpg/.jpeg image file.
"visibility" is how you want your addon to be shown. 0 = Public, 1 = Friends Only, 2 = Hidden.
The "title", "description", and "changenote" fields can be edited how you wish.

Example:

Save the file. Now, it is time to upload the addon!

There are two methods you can try. First one didn't work for me as it didn't want to log in properly but it was probably just issue on my end. This method seems faster and easier so I'll mention it anyway.

Create an empty batch file (.bat) in the folder where you put in SteamCMD. In that batch file, put in this line:

Replace "myLoginName" and "myPassword" with your Steam username and password (Don't worry, this program was built by Valve themselves, so your account will not be hacked if you run this) and replace "myItem.vdf" with the name of your .vdf file

After that, save the file and open up the batch file you made. If you have Steam Guard, enter it in if it asks you to.

If everything went according to plan, you should now have your mod uploaded/updated!

Run steamcmd.exe, once in steamcmd, use the 'login' command to login. You will need to enter your username and password.


SteamCMD may prompt you for a Steam guard authenticator code.
Type in the command it shows up and your code after space.

Finally, use the command "workshop_build_item" to upload your item. Replace "myItem.vdf" with the name of .vdf file you previously created.


If all went well, you can exit steamcmd with the 'quit' command.


Good luck, hope it will work out for you too!

• After using SteamCMD your main Steam app disconnects, you need to restart it.
  
• Sometimes the thumbnail doesn't update but the mod itself does.

I've discovered this guide and used it as reference.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2851702944

btw. Enropy: Zero is pretty awesome HL2 mod, go check it out!