I just initiated my first mega-backdoor Roth IRA rollover, and I figured I'd give a report out since it can be confusing. And even once you get the concept down, you have to figure out the actual *mechanism* in working with your institutions to actually make the rollover. In this case I was rolling over from Fidelity (401k administrator) to Vanguard (IRA administrator).
http://www.chaostheoryblog.com/2016/03/my-experience-performing-mega-backdoor.html