1. Install Node.js
2. Open a console and go to the project directory
3. Write "npm install" to install all Node.js modules needed
4. Build TinyMCE by writing "jake"

jake Runs the minifyjs, less, jshint build tasks.

jake -T List all build tasks.

jake minify Compiles the core classes, plugins and themes into minified versions.

jake less Compiles all LESS based skins into css files that can be included in the browser.

jake jshint Runs all js code though jshint.

jake release Builds release packages with the version specified in changelog.txt.

jake bundle[themes:*] Bundles all themes into the tinymce core js files.

jake bundle[plugins:*] Bundles all plugins into the tinymce core js files.

jake minify bundle[themes:modern,plugins:table,paste] Minifies the core, adds the modern theme and adds the table and paste plugin into tinymce.min.js.

You can read more about how to contribute to this project at http://www.tinymce.com/develop/contributing.php