Here are some pictures from my install.
1. Tap into a fuse box slot that has power, but no fuse. Add a 3A fuse and run the wire behind the panel underneath the steering wheel and then up where the ashtray would be.
Alternatively you can add a new circuit (following this guide:
http://www.myturbodiesel.com/wiki/circuit-addition-to-fusebox-in-your-mk5-or-mk6-volkswagen). A lot more work, but a nicer end result.
2. I had a step down converter lying around from a previous project. Connect the wire coming from the fusebox to the +12V in on the converter. The 12V ground wire only has to be a few cm long.
3. Cut the USB plug cable in half and you will see it contains 4 smaller wires. Connect the +5V and 5V ground from the converter to the corresponding wires in the USB plug cable. Do not count on the color coding being standard, i.e. red wire is power and black wire is ground - do a continuity check! Finally short the D+ (green) and D- (white) wires so whatever you connect knows that it can draw as much power as it needs, i.e. not limited to 500mA. I tested this with several phones and some (such as Galaxy S3/S4) works without shorting while others (such as Xperia Z2) does not.
4. Finally mount the assembly underneath the ashtray.
