Using a Color Checker
Apply paint to the color checker front-to-back rather than from the side.
Assuming your studio is set up correctly to minimize glare, applying paint front-to-back will not produce any glare on the color checker, making it easier to see the color you've painted on. This is how I do it in all my videos. If you apply paint to the color checker side-to-side instead, sometimes the ridges in the paint created by the bristles of the brush will create streaks of glare, making it difficult to judge your color.
If you're having problems with glare on your color checker, watch this video:
If using a shadowbox, make sure the shadowbox light does NOT light the color checker.
If you've made a shadowbox with its own light source (as shown here), it's very easy to determine whether or not the shadowbox light is affecting your color checker. Simply turn ON your shadowbox light and turn OFF your main studio light, then go sit in your chair in front of your easel and hold your color checker out towards your shadowbox as you would while painting. Move the color checker around and if at any point your color checker is lit by the shadowbox, it will be obvious.
If you have never used a color checker before, watch these videos first!
This short video covers the basics, explaining how the color checker works and how to "balance your white":
And this half-hour video is a full demonstration on how to mix accurate colors from life using a color checker, covering things like how to adjust colors when they don't match, what to do when something you're checking is "darker than black", and lots more: