Clone this repo into <ghidra_home>/Ghidra/Extensions/ folder
(don't confuse with <ghidra_home>/Extensions/).
That's it, just run Ghidra and select m16c language.
Install Eclipse, set up a development environment following this guide.
Add GhidraDev Plugin to Eclipse and create a new Ghidra Processor Module following this other guide.
Clone this repo into the newly created module path.
Build and run!