When a Config is saved in Sandbox, a Local version is stored in your game's "data" folder. When you are finished with the Config and wish to upload it to the workshop, perform the following steps.

*** Find your local Config's folder
Navigate to your Garry's Mod installation folder. If you are unsure about how to do this, you can find it by Right-Clicking on Garry's Mod in your Steam Library, choosing Properties and then Local Files. Click the Browse Local Files button.

From here, navigate to garrysmod/data/nzombies-unlimited/localconfigs/{your config's codename}.

*** Convert all files to .lua
Change the file extension of all files found in the folder to .lua.
Image: https://i.imgur.com/Del7yPc.png
If you do not see file extensions, you will have to enable this first (Google how for your own operating system).

This step is needed as the Workshop do not allow .txt files in addons. The gamemode will still read the file as pure text, so nothing needs to change besides the file extension.

*** Make your Addon Folder
Follow the general steps of Workshop Addon Creation[wiki.garrysmod.com] to create your Addon folder. Inside it, make the following folder:
Move your Config previously found in data/nzombies-unlimited/localconfigs to this folder.

*** Add a Thumbnail (Optional)
If you want to add a thumbnail to be displayed on the Main Menu as well as the Config Loader, create a 16:9 image using your preferred image editor. Save it by the same name as your Config's folder (the Codename) and as .jpg. Make a new folder path in your Addon folder:
and put the .jpg image in here.

Note that this only works for Workshop Configs. If you want the Thumbnail for your Local Config, you should just place the .jpg image in the local config's folder (as found earlier). Just remember to move it to the above when you create your Workshop Addon for the Config.

*** Upload the Addon
Simply follow the remaining steps of the Workshop Addon Creation[wiki.garrysmod.com] guide. After upload you may choose to update your Config with the Workshop ID associated with your addon. (This currently does nothing, but will later allow users to view the Config's addon from in-game).

Configs are a folder containing save information about the map you have created. It is what represents a "level" or "map" in the gamemode (not to be confused with the actual map itself). A config contains the following information:

• A name, description, authors, and list of required addons
  
• A reference to a map
  
• A file containing information about every single entity's location, angle, model, etc.
  
• A file containing every setting set in Extension Settings

You start Sandbox on the map you wish to create a Config for. Ensure that the console variable "nzu_sandbox_enable" is set to 1 (it is set to 1 by default, but if you changed it to 0 to disable nZombies in Sandbox).

Navigate to the nZombies Unlimited tab in the Spawnmenu (Q). This menu contains a set of sub-tabs. The Save/Load tab is where you create, save, and load your Configs. Click the Create a new Config... button and enter a Codename (see next question). Now simply build your map using the Spawnmenu and the new nZombies Tools and use the Save Config button to save it.

A Config has a set of metadata determined by the Save/Load tab in Sandbox.

• Codename:
  This is the name of the folder when the Config is saved and the identifier of the Config. Make sure this is unique, as any two configs using the same Codename will conflict! The Codename is set initially when pressing the "Create a new Config..." button and can only be changed by manually going and renaming the folder in your local configs folder when Garry's Mod is closed.
  
• Name:
  This is the name that is displayed on the Config. It can be anything, but make it relevant and not too long.
  
• Authors:
  This is the list of Creators you wish to give credit for this Config. It can be anything as well, and doesn't necessarily have to be your Steam name.
  
• Description:
  A description of the Config. In Call of Duty, this is the Mission Briefing.
  
• Required Addons:
  A list of required addons. The Config will warn about these before being loaded, allowing players to subscribe to these addons. It also appears in Mismatch.
  
• Thumbnail:
  The image displayed for the Config. This is something you will have to make outside the game. Read how in the last step of the How to share Configs on Steam Workshop guide above.

Sandbox is used to build the Config, but to play it, you will launch the nZombies Unlimited gamemode. From the main menu, you can access installed Configs through the "Load Config..." button. Note that this button is only unlocked for Admins of the current server (you are always admin in your own game). Click a Config to display it on the Menu, then go back and the top button of the main menu should now say "Load Selected Config".

Clicking that will freeze your game for a moment as it is loading the Config and spawning the entities. If the Config is of a different map, it will change map first. The button should now say "Ready" and clicking that will initialize the countdown to begin the game on the current Config.

The menu will automatically open when you Unspawn, and will automatically close when you Spawn In. You cannot close the menu when you aren't in-game and no one else is playing.

The menu is toggled with the F1 button. If this doesn't work for you, you can alternatively toggle it with the console command "nzu_menu".