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. Let's look at the art description for Goblin Bushwhacker:
Color: Red creature
Location: Rocky mountainous area
Action: A goblin warrior leans precariously headfirst off of a cliff face or large boulder, ready to ambush someone out-of-frame. The goblin has a sword, spear, or other long melee weapon, and has precariously anchored himself to the rock face using ropes and spikes.
Focus: The goblin warrior
Mood: Upper ground advantage
Next, Mark submitted a sketch to show the creative team how he planned to interpret that description. Here's the sketch he sent:Goblin Bushwhacker sketch by Mark Tedin
There's the goblin, ready to ambush his quarry. And on the right, you can see where Mark worked in a little bit of storytelling: this particular goblin has been bushwhacking people from this particular spot for a while, and he's keeping a tally. The creative team gave Mark the thumbs-up to proceed.
Final ArtGoblin Bushwhacker by Mark Tedin
And here's how Goblin Bushwhacker looks when it leaps out of packs of Zendikar.