Modification

As MM8BDM is created in Zandronum, it shares its extremely flexible modding capabilities. Creating your own levels, gameplay modifications, skins etc is as easy as can be... once you know how to do so.

Running Modifications

Skins

Skins can be installed simply by following these steps:

Create a new directory in your MM8BDM folder called "skins".
Extract the wad/pk3 from the skin zip file into the new skins directory.
Launch the game. You should now be able to select your skin from the player setup menu! (NOT the in game selector)

You can use skins online but only those who have the same skin installed will be able to see them.

Everything Else

There are several ways to launch MM8BDM with mods.

MM8BDM Launcher - Using the official launcher you can use "Add File" to add mods to the game. (Windows only)

DoomSeeker - If a server is hosting some modification(s) you own, you can connect to it easily!

ZDL - A very flexible little launcher that can easily launch a list of mods of your choice. (Windows only)

Developing Mods

Due to the open-pk3 nature of MM8BDM, it is fairly easy to create mods for. Just about any aspect of the game can be modified, from something as simple as a new skin or music track to something as complicated as a customized game mode or completely new playable character with custom moves.

Here are a few resources to help get you started.

Tools

Slade is an all-purpose pk3 and wad editor. You will likely be using this to develop the bulk of your mods.
Ultimate Doom Builder is a multi-faceted map editor for Doom.

Forum Tutorials

Custom Skin tutorial - A tutorial for creating your own skins using SLADE.
Custom Bot tutorial - A tutorial for creating your own bots.
Custom Weapon Template - A weapons template with tutorial for creating your very first weapon!
How to create your own Music WADs - A quick tutorial on how to put your own music in the game.
Doombuilder and You: Mapping Essentials 20XX - A very thorough tutorial on creating maps using Doom Builder. Includes setting up GZDoom Builder and Ultimate Doom Builder up for mapping with MM8BDM.