Create your first mod

tl;dr:

Open a Command Prompt/terminal (on Windows, I suggest installing Git for Windows and running that through Windows Terminal)
Run dtmt new <path/to/new/folder> and fill out the prompts
Edit the file scripts/mods/<id>/init.lua
Run dtmt build, which will produce some files in out/
Run dtmt package, which will produce a zip file
Import that zip file in DTMM and deploy it
Check Continued Development below

In a Command Prompt/terminal, run dtmt new <path/to/new/folder> to create a skeleton structure from the default template. During this, DTMT will prompt for several fields, including:

name: The human-readable name of your mod. This will show up in various places like DTMM's mod list or DMF's mod options view.
id: A unique ID for your mod. The default is generated from name and should be good enough in most cases (clicking Enter without typing accepts the default value). This will be used throughout your code (e.g. in get_mod() calls).

DTMT will now create the given directory from the mod template:

<path/to/new/folder>
└──<id>
   ├──.luacheckrc
   ├──dtmt.cfg
   ├──packages
   │  └──mods
   │     └──<id>.package
   └──scripts
      └──mods
         └──<id>
            ├──data.lua
            ├──init.lua
            └──localization.lua

You're now ready to start editing the init.lua file. For starters, you might want to just add a simple print("Hello, world!"), which will print this string in your console logs once you've loaded the mod.

With the file edited, you now need to turn your mod into something that DTMM can load into the game. To do that, navigate your terminal to your mod's folder (where the dtmt.cfg is) and run in sequence dtmt build and dtmt package. The first command will compile your mod files into game bundles, the second will package things into a single <id>.zip for distribution. Watch the output for any errors, but if all goes well, you can then import that <id>.zip fiel into DTMM and deploy.

Continued development

After you've verified that your new mod has been loaded in the game, you will want to start the actual implementation of your mod. Since it would be rather tedious to create a new zip file, import that in DTMM and deploy it for every change, there is one more command to learn: dtmt watch --deploy <path/to/Warhammer 40k Darktide>.

This will make DTMT continuously watch for changes in your mod's files, re-build the mod bundles and immediately deploy them to the given game directory. The path given should be the directory above bundle/, e.g. C:\Program Files\Steam\steamapps\common\Warhammer 40k Darktide.

But there are two limitations, since DTMT cannot do a full deployment like DTMM:

The mod has to be deployed once through DTMM first
It only works as long as the list of files doesn't change. I.e. if you add a new file to your mod, or remove one, you will have to deploy that change through DTMM again, once

Dev tools

Lua

DTMT will already run a basic syntax check on your Lua files. For additional static analysis, install luacheck and integrate it with your chosen editor. selene exists, too, and might work as an addition for its extra lints.

Wiki

Building the source code
Obtaining the Oodle library

DTMM

Installing mods
Register DTMM as handler for Nexus downloads

DTMT

CLI Reference
Building your first mod
Migrating from loose files
dtmt.cfg Reference
Murmur hashes and dictionaries

Reverse Engineering

Getting Started
Sound
Textures

File Types

Status
Bundle
Bundle Database
Texture
Material
Strings
Package
Wwise Event
Wwise Stream
Wwise Bank