Final Piece: [link]
Note: I use Illustrator CS5
1: Look for inspiration
2: Sketch out the rough line of how you want the picture to be.
3: Line it out in black and make adjustments where needed.
4: Add details;
-- Such as the grass, the wood texture, the road etc.
5: Colour with random colours;
-- Adventure Time tend to have unique colours so splashing whichever colour you like on the drawing would be fine.
-- As for the shading, Adventure Time Style often have general curves of a darker shade for shadows.
Hope this helps C: