Modding the game still requires you to copy and change files into the right directories. There is no graphical user interface for changing unit stats, looks etc. in the game. What you can find in the game now, however, is an interface to test and publish Mods you created, and to activate Workshop Mods in unranked Multiplayer lobbies.

If you want to start making your own Mods, you first have to create a folder called "mods" inside the content-folder of your Tooth and Tail installation, e.g.:
C:\Program Files (x86)\Steam\steamapps\common\ToothAndTail\content\mods
Inside the folder "mods", you need to create a subdirectory for each Mod you are working on. Give it any name you want - as an example, we'll call it "mymod", now.

Now here's what goes inside a specific Mod's folder (e.g. the mymod folder):

• All the things you want to change!
  You need to organize them in the same way as the files/folders you find in the game's content folder. For example, if you want to give Pigs a minigun, you copy the file pig.xml
  from ../ToothAndTail/content/data/multiplayer/actors/other
  to ../ToothAndTail/content/mods/mymod/data/multiplayer/actors/other,
  and replace the trait "pistolpig" with "minigun". - Done!
  You only need to put the files you want to change into your mod's folder. That will keep the mod small, fast to download, with easily recognizable changes.
  
• One file called mod.xml, containing some meta-info about your Mod. It needs this structure:
  
  <?xml version="1.0" encoding="utf-8"?> <ModInfo> <PublishedId>0</PublishedId> <ModName>My Mod</ModName> <ModDescription>A Mod that gives Pigs Miniguns.</ModDescription> <ModAuthors>Me (and that guy who wrote this guide)</ModAuthors> </ModInfo>
  Keep PublishedId at "0" for a new Mod (it will be given an ID once it's published)! ModName is the name that will be shown on the Workshop and when you select the Mod in game.
  
• header.png - the picture that will be used on the Workshop page, e.g. a Pig with a Minigun.

Please enter a short description of your Mod's changes in ModDescription! This description will pop up just before deck selection. It should give players an idea of what to expect without reading a long text. If you changed more than two or three details, keep it abstract! Players can still find out unit stat changes during deck selection or even while the game is running. If you want to provide a full list of changes, you can simply add a separate text file (e.g. change_notes.txt) and include it in the Mod's directory.
You should, however, inform players about hidden changes, e.g. if you made each farm yield 5k Food (instead of 500), because that might be hard to find out - and playing without such knowledge against someone who knows would just be annoying.

For now, you can change multiplayer content, e.g. units and structures, general balance data etc., and textures with Workshop Mods. It is mainly meant for balance Mods and additional units. Preferably change textures only if needed for a new/changed unit - for purely aesthetic reskins of your units, its easier and better to change the textures in your local installation, directly. That way the reskins won't affect other players.
Here's a guide how the data manipulation works:
https://steamproxy.net/sharedfiles/filedetails/?id=608983584
Experiment! - And feel free to ask me about particular changes you didn't get done yourself.

Once everything is prepared, start the game, select the new "Workshop" category in the Main Menu, then "Test and Publish Mods".*
This will create a new offline game, in which you can activate offline Mods (this is the only way to select offline Mods). In order to activate Mods, go to Match Options (during Commander selection), then Mods, and you will see a list of offline Mods on your PC. Select one, the screen will freeze briefly while loading the new data, and the Mod is activated.

Start the game to test your Mod. When you're pleased, press the Pause/Esc button. In the menu you will find a button to "Publish (your) Mod" (or "Update", if it has already been published). Press the button and after another short freeze, the Mod should be uploaded.
It may take a few minutes for the Mod to become visible on the Steam Workshop.** You have to subscribe to it, before you can activate it in other (multiplayer) lobbies.

*) This option will be locked in the public_test version of the game, because Mods can cause trouble on older versions of Tooth and Tail (to which you will likely switch back during the testing phase). If you are up for testing this, anyway, and you are not afraid of some extra work in order to avoid trouble with Mods, contact me (EELuminatus)!

**) In the public_test version of the game, Mods you uploaded will be visible for friends, only, in order to prevent unsuspecting players from downloading Mods that are not supported on their version of the game.

In any unranked or offline multiplayer lobbies, the Mods you subscribed to on the Workshop can be activated the same way offline Mods are activated in Mod test/publish lobbies: select Match Options during commander selection (only possible for the lobby owner) > Mods > select your Mod.

A Mod stays active until you select another Mod or leave the lobby. Players who join your lobby will automatically download and activate (but not subscribe to) the Mod you activated.

Many things are possible, already, e.g. completely changing units, weapons, structures etc., adding new ones, changing costs, build times, and more.
A few things, however, are not (yet) possible, because it would require reloading more data, in different ways. Here are some features you might expect, which are unfortunately not supported:

• changing singleplayer content (that's planned in the future, however)
  
• changing things in the "core" folder (e.g. faction colors)
  
• adding new status effects (the mechanics behind them are hard coded)
  
• adding custom audio cues (because TnT is using a sound effects engine that requires compiled sound banks - that makes it way harder to add new sounds on the fly)
  
• localization, e.g. for new units (new localization strings need to be defined in the game's code, adding to the files in the strings folder and referencing them in the data is not enough)

If you're trying to get something done, but it does not seem to work at all: don't hesitate to ask whether it is actually possible.