Step 1 - Selecting a neutral area of the image
Use the Color Sampler Tool and pick a neutral point in the image, preferably as close as possible to a middle grey. A grey card in a frame shot in daylight is ideal, if not you can try neutral clothing or pavement.
You can skip directly to Step 3 if you want to balance the image by eye.
Step 2 - Checking RGB balance
You can check the current balance of your selected point by activating the Info panel from Window - Info.
Step 3- Adjusting the Y and M layers
These layers simulate adjusting a color head's filters in traditional printing, as well as the way the software works on Fuji Frontier scanners.
By adjusting the layer opacity you can add both Yellow and Magenta to the image.
The M layer should be used first to match the R and G values. Use it if you see an overall
The Y layer will control the levels of the Blue channel. Use it if your image has an overall
Why this method and not another?
This method is fast when you get used to it, and more accurate when done right.
It's also a good idea to remember the Y and M opacity level in % to apply to other images shot in the same conditions to achieve the same balance.
If you want to use the grey point picker in a Curves adjustment, you need to first merge all layers for best results.