Artist Steve Prescott has illustrated 58 Magic cards, and they're all cool.
The first step of the creation of a card's art is its art description. The art description tells the artist what the card will do, what its flavor is, and what the mood of the illustration should be. Here were the instructions given to Steve for Enlisted Wurm:
Color: Green and white creature
Action: A wurm of Jund (styleguide p. 63) has been 'trained' and armored by the knights of Bant. Now it's their siege weapon. Show this wurm rearing up to smash through a decrepit necropolis. Several human Bant "handlers" hold tethers tied to the wurm and they're using them to try to keep it pointed in the right direction.
Focus: the armored wurm
Mood: their control over it is tenuous at best.
The next step is for the artist to submit sketches of his or her vision of the art description.
Here's Steve's initial sketch submitted for the wurm:
Isn't that a great sketch? It's clear, precise, and easy to envision the final piece. There were no changes requested, and the final version looks exactly right:
And here's the card in its final state, as you can find in booster packs of Alara Reborn!