1) Download Steam Workshop Uploader:
https://github.com/nihilocrat/SteamWorkshopUploader/releases/tag/Build10

2) Unzip downloaded archive.

3) Inside archive you will find steam_appid.txt.

4) Open it and change Game ID to RealRTCW one: 1379630

5) Organize your content inside WorkshopContent folder

5.1) Create separate folder for your map/mod inside WorkshopContent folder.
All of your .pk3 and custom .dlls should go there.

5.2) Create simple .txt file and name it as description.txt. Inside that .txt file specify name of
your mod/map. It will displayed in the "Addons" menu in-game. description.txt goes into your mod/map folder.

5.3) Create a preview image for your mod. Recommended size and format is 512x512
PNG. Put the image in the WorkshopContent folder.

6) Launch SteamWorkshopUploader and upload your mod. You can update your mods/maps from there as well.