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 Wayne was given for the card that was named "Goblin Explosioneers" at the time:
Color: Red creature
Location: Battlefield or mountain setting
Action: Show two goblin soldiers who are in charge of the "bang-chucker," an explosive-hurling contraption that you are free to make up. Perhaps it's a wide device that yolks across both of their shoulders with the flinging structure between their heads. maybe one of the goblins peers into an aiming-glass on the device, waiting expectantly with his hand out for the ammunition, and the other goblin fumbles with a spiky metal ball (or a wad of explosive wrapped in butcher's paper held together with twine with a wick coming out of it) that looks like it's about to explode at any moment. (It's a cartoony situation, but keep the style relatively straight.)
Focus: The two goblin "explosioneers"
Mood: Incompetence mixed with high explosives -- a goblin specialty.
With that art description to work with, Wayne submitted a sketch to the Magic creative team:
The creative team liked it, although they wanted to make sure that the final version wasn't too cartoony. Aside from that, the tone was perfect. Let's go!
Here's the final card as it appears in Magic 2012 booster packs: