I recently was introduced to the "Auto adjust Kelvin + G/M" tool by Danne while doing some testing on his new MLV workflow. This is such a valuable tool and we should be using it more often. Basically the way Canon tells you to do a custom white balance is by shooting a still of a white or grey card then using that captured image to set the white balance. Those steps might make sense if you have a mirror flipping in front of your camera's sensor but since the EOS-M is mirrorless there shouldn't be a need to capture a still that you're going to throw away, especially if you shooting in crop movie mode.
Ok--so what I did was to make an extra copy of the "Auto adjust Kelvin + G/M" tool in the main shooting menu to make it more accessible. I put it in my EOSM__working branch that you can build from
source via bitbucket or
download. I also made a
pull request so you can comment on it.
Here's how it works. The tool actually a submenu under White Balance in the Shooting Menu.

This makes it a bit difficult to get to with the EOS-M so I put an extra copy in the main shooting menu.

Aim at a white or grey card and it will sample the center 200 pixels to adjust the white balance in both the amber, blue and magenta green.

That's an Xrite Color Checker Passport though there are
other options. Don't worry about focus, just make sure it is in the center.
By the way, all professional camcorders have a button dedicated to doing this--the EOS-M has a lack of buttons but this makes it much easier. No more excuses for not taking the time to set white balance before shooting!