There are different ways to make and upload mod in different creative workshops. Some of them are made and uploaded directly in the game: for example, the built-in map editor of Portal 2, or some need special software to make or upload: for example, workshop publisher, the elder of Garry's Mod Scroll:Skyrim The details of creation kit can be found in the respective workshop of the game. It should be noted that descriptions, names, pictures, etc. may also need to be prepared when submitting mod.