So, I decided to make this to show you guys how I shade my cubes. For the purpose of easily viewing the difference and seeing what goes where, I used different colours.

Pink - Top/Lightest shade

Blue - Left/Middle shade

Orange - Right/Darkest shade

I also drew out larger versions of the pixels to show you how they should look.

The grey cube at the bottom shows the same cube as above, but I replaced the colours with the colour I selected (grey). If it helps, you might want to start out by using different colours to differentiate the different shades, then replacing them after.

I hope this helps!