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 Efrem was given for Slice in Twain:
Color: Green spell
Action: We see a Golem that's just been cleanly sliced through by a glowing elvish sword. The sword is wielded by a "steelbane," an elvish warrior who hunts down artifact creatures.
Focus: the fact that the golem is destroyed in one blow
Mood: Prowess with a blade combined with a shaman's magic
Armed with that art description, Efrem submitted a sketch to Magic's creative team.
The creative team liked the action in the piece, and sketch was approved. When the color came in, the final version really popped.
And here's what it looks like in Scars of Mirrodin packs: