I just figured this out. First off, there is a tutorial. If you haven't read it., read it. It can still be a little confusing. However, it isn't that hard once you understand it.
1st step is creating the mod .json file by creating a new mod in the mod building without adding any assets to it. Just type in the mod information, and save the file settings for the mod. This should create a .json file to track the mod. The Json file is critical to create and include in the bundled mod file, as it is the main meta data tracker it seems.
Next, click the asset window button to open it up. There should be the json file listed under the assets. If not, without anything selected in the unity editor, click save file settings and then click add asset. Again, you should see the Json file included in the mod builder asset window now.
Next add the assets you want to bundle into your mod, click build, and select where you want it to compile the final .dfmod file. It will create three folders, one for each main operating system. It will compile the final mod in the one your computer's os is running.
Some side notes to consider, if the mod has any script files it uses to manage art or sound assets, you may need to modify the script files if you also modify the art assets. Many times, it isn't as easy as just swapping out an asset and repacking it.
viewtopic.php?f=22&t=277
Simple Flow Chart
Mod Builder -> Mod JSON File (Must have to track mod assets and data) -> Assets -> DFMod
