Basic modding guide:

Modding Guides on Steam: http://steamcommunity.com/app/244850/guides/
Official Modding Forum: http://forum.keenswh.com/forums/modding.325599/
Modding Community: https://steamcommunity.com/app/244850/workshop/

IMPORTANT: Don’t forget to back up your game folder before making any changes
\Steam\steamapps\common\SpaceEngineers

MODDING API

The Modding API brings a lot of new possibilities to modders and allows them to alter the game by writing C# scripts which have access to in-game objects

For more info and instructions please read our Modding API Guide.
Modding API forum: http://forums.keenswh.com/?forum=408074

SPACE ENGINEERS ModSDK

Space Engineers – ModSDK, is an application that includes a set of tools specifically created for modding. The application is totally free for everyone, even for those who don’t own Space Engineers game. In order to find it, open the Steam client, go to Library, Tools, type “Space Engineers ModSDK” in the searchbox and just follow the steps to install it.

Space Engineers – ModSDK includes the following modding tools:

• MWMBuilder – this tool can be used for converting graphic assets to Space Engineers format
• VRageEditor2 (ModelViewer) – can be used for previewing models without having to be in the game. Modders can view models, textures, dummies, dummies axis, physical shapes, axis, bounding box and material setup. More info: Model Viewer Guide
• Texture packing tool for creating compressed “channel packed” textures – Official Guide
• original models, sounds and music from Space Engineers game
• tools for converting sounds and music

Note: textures are not included because they are already in the game
ModSDK will be improved during time and more features will be added

SPACE ENGINEERS – Visual Scripting Tool

Visual Scripting Tool was designed to allow users to design game scenarios for Space Engineers without needing previous coding experience. Mod developers can utilize this tool to create scripted gameplay events, play cutscenes, or modify game values at runtime.

• Official Guide

MODDING ACADEMY

A series of video tutorials that will initiate the players into the secrets of modding in Space Engineers

• EPISODE #1: Skybox

SAVE FILES / WORLD FILES

Every world you generate is saved in its own folder that can be found in
c:\Users\{YOUR WINDOWS USERNAME}\AppData\Roaming\SpaceEngineers\Saves\{Your Steam ID}\

File types:

• Sandbox.sbc – basic description of a world
• SANDBOX_0_0_0_.sbs – detail definition of a world, positions and states of objects (we don’t have more details at this moment, but it should be pretty self-explanatory – run some experiments)
• .vox – binary voxel data for an asteroid (we don’t have more details at this moment)
• .xmlcache – don’t modify these files, they are cache files regenerated on each save

MOD FILES LOCATION

Each mod has now its own folder located in \Users\{username}\AppData\Roaming\SpaceEngineers\Mods\{mod name}

The mod folder has to contain a sub-folder called Data with definition files of added or updated mods. Everything can be replaced/updated by the modder (audio, video, textures, models, gui, etc).

SHADERS

Files with FX and FXH extension are vertex/pixel shaders for various rendering operations and can be found in steamapps\common\SpaceEngineers\Content\Effects2\

Every shader file gets recompiled on game launch. If you modify one, restart the game. If the game crashes on launch, it’s probably due to a compilation error caused by your changes.

TEXTURES

Almost all textures are in DDS format (DXT compression). You will need an editing program that can manipulate them. If you use Adobe Photoshop, we recommend installing this plugin: developer.nvidia.com/nvidia-texture-tools-adobe-photoshop and use these settings for exporting:

Textures can be found in: steamapps\common\SpaceEngineers\Content\Textures\

There are multiple categories of texture files: GUI, particles, lights, models, voxels, etc.
Model textures with “_ns” in their name contain “normal map” in RGB channels and “specular” in ALPHA channel. 
Model textures with “_de” in their name contain “diffuse” in RGB channels and “emissivity” in ALPHA channel.

GAME DATA

Game data is stored in folder: steamapps\common\SpaceEngineers\Content\Data

The folder contains the definition files for Cube Blocks, Physical Items, Voxel materials, Transparent materials (particle effects), Components, Blueprints and a few other things. By using the XML files here (extension .sbc) you can modify most of the game objects and add, for example, a new type of cube block. When modifying files, make sure you delete the .xml cache file. It is possible to create, for example, round armor blocks, a new type of thrusts, gyro, a different cockpit and many more.

3D MODELS

MwmBuilder is the tool for building mwm models from fbx files.

Note: You may need administrator rights to copy or create files in Program Files folder; you can copy MwmBuilder and dlls into a different folder.

MwmBuilder Location:
{Steam installations directory}\SteamApps\common\SpaceEngineers\Tools\MwmBuilder.exe

Models are stored in: steamapps\common\SpaceEngineers\Content\Models\

SIMPLE USAGE:
1) Copy FBX models to the same directory as MwmBuilder.exe
2) Create an XML file with the same name as the model. Giraffe.fbx will have Giraffe.xml.
3) Run MwmBuilder.exe
4) You’ll find .mwm files in the same folder

For advanced usage see here: Advanced 3D Models Guide

Moddable collision models: Adding new model with collision geometry