The following steps work on a fresh install.

Install node
Download the current version of node and npm from nodejs.org
Add the directory containing node and npm to your path.

Clone motion-path-js
$ git clone https://github.com/Motion-Path-Polyfill/motion-path-js.git
$ cd motion-path-js

Get dependencies and build tools
$ git submodule update --init --recursive
$ npm install

Build and run tests
$ npm test
To watch for file changes and automatically retest, use:
$ npm run watchTests

Use the polyfill
Add third_party/web-animations-js/web-animations.dev.js and motion-path-polyfill.min.js to your web page.
Include offsetPosition, offsetPath, offsetDistance, offsetRotate and offsetAnchor in your keyframes when calling element.animate. Example.