How to remote control Youtube from an iPhone?
On your PC/Mac, go to http://youtube.com/tv  (that's not your usual URL, it's the "Lean Back" mode)
Go to settings -> Pair a Device (on the sidebar, on the PC)
Once instructed to do so, go to your phone and just navigate to http://youtube.com/pair ... if you have the YouTube app, it will ask if you want to open on that app, say yes - if you don't - just move forward
Put the pair code from your PC into the pair box on the phone


Presto - you're driving from your phone.

Essentially, "Lean Back" mode treats your PC like a TV, that's why it works.