This requires that you atleast know how to open the developer console, if you do not, follow this guide first, then come back to this one: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=627640216
You also have to be the host of the current map/server in order to assign a new skybox.

1. Launch an map that doesn't support skypaint.
(can be any map from the Half-Life series, TF2, L4D2, DoD or CSS)











2. Open up the developer console and type following command "sv_skyname painted"




Your skybox should turn black now.











3. Spawn the sky editor found in the entities tab-->Editors







4. Now you can edit the skybox to your hearts content