Install

Install Git
Install Node.js
Install needed mobile SDKs
On Windows, as explained in node-gyp documentation:
- Install Python 2.7.x. Python 3.x.y is not supported
- Install Microsoft Visual Studio C++ 2013 (Express version works well)
Execute $ sudo npm install cordova --global once (don't use sudo on Windows)
Execute $ npm install, then wait...
You should now have everything you need to get started (those folders listed in ./.gitignore for example)

Run in your default Web browser (Chrome is better)

Execute $ npm start and enjoy ;)
It is possible that you'll be prompted to choose an IP address or localhost, pick the one that suits you better
A browser window should now open automatically, if it doesn't, go to http://:8100
Now everytime a file located in ./www is updated the browser will refresh, how cool is that?

For iOS, execute $ sudo npm install ios-deploy --global once (don't use sudo on Windows)
Setup your devices for development use first, and hook them up to your computer
Execute $ npm run ios and/or $ npm run android and enjoy ;)

For iOS, execute $ sudo npm install ios-sim --global once (don't use sudo on Windows)
Execute $ npm run ios:emulator and/or $ npm run android:emulator and enjoy ;)
You can start a specific emulator using $ npm run <platform>:emulator -- --target="<target>"

Execute $ npm run build. That's it, you now have a ./build/ directory containing the built app and this directory is automagically pushed to the phonegap-build Git branch