Artist Svetlin Velinov has illustrated nineteen Magic cards, going back to Worldwake.
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 Svetlin was given for Titan Forge, including a lot of location details:
Color: None (artifact)
Location: strange void in the Quicksilver Sea
Action: From 3/4 overhead we see a spot in the Quicksilver Sea where the mercury-like liquid simply refuses to flow -- a large, circular space where the "floor" is the silvery sea bed and the "wall" around the space is quicksilver suspended as though by a forcefield. Within this spot stands "the titan forge," a huge device for creating towering metal colossus after colossus. The cool colors of the Quicksilver Sea and sky above contrast with the semi-molten metal of the forge, which suggests the faint outline of a huge, imposing humanoid figure at least 30 feet tall.
Mood: With enough mana to power it, one could create an army of enormous soldiers.
With that art description to work with, Svetlin submitted a sketch to the Magic creative team:
The creative team's comments were all variants on "This is good," so Svetlin was given the go-ahead to proceed:
Here's the final card as you'll see it in Mirrodin Besieged booster packs: