What is ETTR:
"Exposure-To-The-Right". A technique used for best utilisation of the camera sensor, to get best output with least image noise, while avoiding overexposure.
A compromise:
It does not always give you least image noise. Sometimes, a camera in its general preset settings can decide to overexpose, therefore brighten the noisy/shadowy parts of the image, while blowing highlights. ETTR is there to show you the most real preview on this compromise, and allows you to decide which way you want to go about it.
In some cases you gain on the noise front, sometimes you save yourself from blowing highlights, and with high dynamic range scenes, there is often simply no help anyways, and bracketing will give you better result.
Settings:
Most Canon cameras act the same.
1) Make a totally overexposed white photo
2) Select custom white balance (best in your camera menu).
3)Choose the overexposed image as a custom white balance preset.
4)For best results, try to shoot lowest standard range ISO possible. Preferably ISO 100.
Check:
The image preview/LiveView feed should have very strong green tint.
Usage:
Turn on RGB histogram in your camera menu.
The histogram will now show you best estimation of true sensor exposure. Don't let the histogram to show you large column in any color channel on the most right side of the histogram. Best practise is somewhere between little to no data in the most right pixel of the histogram. You should check the results in your camera software, as even this approach is not flawless.
Once you load the image in your editor, obviously, straighten the white balance, so it looks as intended.
Tip:
You can still allow very slight overexposure of green channel, as it is the strongest channel, and small overexposures can be retouched in post.
Based on my experience, more often than not, you do not want to overexpose red and blue channels.
Feel free to add tips, corrections, questions and such. Just giving some love to the M.