How do you manually focus a 5d3 while shooting video?

I've become pretty comfortable with still photography with the 5d3 and now I want to explore the video capabilities. As if you can't already tell, I'm a complete newbie when it comes to videography.

My current stumbling block is: How on earth do you manually focus a 5d3 while shooting video?

So autofocus isn't a good option for a number of reasons. If I'm not shooting at the lens' max aperture, the video would suddenly go really bright as soon as I press af-on. Regardless of aperture, having the video jerk into focus while emitting a "click-click-click" sound spoils the mood of anything I want to film.

I was hoping to manually focus using a technique similar to using Live View while shooting a static shot and having an area magnified to 5x or 10x for focussing. However, the magnifying glass button seems to do nothing when I press it during video recording. My eyes aren't good enough to critically focus based on what I see on the camera's small LCD... so how is it done?

Thanks in advance.