Before the uploading, you need to do the next steps:

SteamCMD is a utility for working with Steam through the command line. You can download it from the official website. It also contains instructions for installing the program for different systems.

In my case, for Windows, I just downloaded a .zip file and unpacked it.

Create a new item in the workshop using the interface provided by the game developers. But it isn't necessary to make it publicly available or confirm the interest.

In my case, I uploaded the sticker to the CS:GO workshop.

Prepare the preview you want to set for the item. The file must be one of three types: JPG, PNG, or GIF. The system lets you using other types, however, they won't be displayed in the workshop. In addition, the file can't exceed the weight limit for a particular game.

In my case, I created an animated GIF and compressed it, because the weight limit for thumbnails in CS:GO is 1MB.

Let's proceed uploading:

Create an empty file and open it with any text editor. Fill it out using the following template:
Where instead of ellipsis, you should enter the fields:

• appid — ID of the game. You can find it out, for example, by going to the game page and copying from the address https://store.steampowered.com/app/XXXXX the number in place XXXXX. This field is required.
  
  
• publishedfileid — ID of the item. You can find it out, for example, by going to the page of a previously uploaded item and copying from the address https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=XXXXX the number in place XXXXX. This field is required.
  
  
• contentfolder — path to the item. Without this field, previously uploaded item files to the workshop will remain the same.
  
  
• previewfile — path to the preview file. This field is required.
  
  
• visibility — value for access. If it is equal to 0 - the item will become public; 1 - the item will become available only to friends; 2 - the item will be hidden from everyone; 3 - the item will be available only through the link. Without this field, the current access won't be changed.
  
  
• title — new name of the item. Without this field, the name will remain the same.
  
  
• description — new item description. Without this field, the description will remain the same.
  
  
• changenote — description of changes in the new version. Without this field, the description in the workshop will be empty after upload.

Save this file with any name and extension .vdf.

In my case, I removed all unnecessary lines and got this:

"workshop" { "appid" "730" "publishedfileid" "2944572057" "previewfile" "C:\Folder\preview.gif" }

Note: if you have any problems, try writing paths to file not with one backslash, but with two. For example: C:\\Folder\\preview.gif

Run SteamCMD and when Steam> appears, enter the following line:
Where:

• USERNAME — login to your Steam account.
  
  
• PASSWORD — password of your Steam account.
  
  
• STEAMGUARD — Steam Guard authenticator code. If you don't have two-factor authentication on your account, you don't need to enter anything.

Enter the following command into the console:
Where PATH is the path to the file created in the first step.

In my case, I created a VDF file directly in the SteamCMD folder, so the path to the file consisted only of its name:

workshop_build_item file.vdf

The appearance of the inscription Success indicates that the download was finished.

When the uploading is complete, you can go to the item page and see the result. To exit SteamCMD, close the window or type quit into the console.

In my case, I've got this item as the result:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2944572057
You can also read the official documentation on the Steam functionality described in the manual if you want.

I hope this guide was helpful, you can also ask any questions in the comments.