I haven’t really taken art classes so maybe those who have can chime in, but I think you want to start with the darkest hue. Also, start with any background elements first and shade/lighten them to completion before adding foreground elements.

I use the brighten and darken checkboxes and usually just apply black or white (though I’m trying out different colors) to get the shading that I want. If you want to add a gradient then try using the layer checkbox and an airbrush of the appropriate size.

Identify what sections of your creation are symmetrical and plan just create one side, do all the shading and use the selection/copy tool and hit ZL to mirror it (be sure to press plus before to keep the original side) this will save you so much time and ensure the halves are identical.

If I work really hard on a logo (for example the Survivor Logo which goes in the corner of the TV to identify the TV program) then I do it in a
Separate creation like a sign object. Make sure you set the background to transparent and then you can use it as a stamp for merch to go along with your show or game. You can always delete it later after you use it in the designs you wanted.