>>7589681
I would start with the black & white / grayscale stuff from the early D&D books. Larry Elmore and Jeff Easley did a whole pile of illustrations to illustrate spell lists, tutorials, etc. Look up Aleena the Cleric or D&D redbox art. Copy a few.

The same artists painted many D&D scenes in color but in a very similar style; it makes it easier to see how that artwork grows from the simpler black and white drawings.