There's a lot of ways to get to an illustration but the sure-fire method is with drawing it first. Illustrators come in all sizes, shapes and varieties. They are as unique as the artist rendering them. I happen to enjoy the linocut method off carving a block and printing it. Now that is definitely old school but if you look around there is quite a bit of 'old-school' methods like letterpress textures showing up in illustrations that are ultimately manipulated in computers.
This Rooster and Tornado were conceived from thumbnails, cartooned out, cut in linoleum, printed and ultimately 'colored' in the computer. All to become spot illustrations in a children's book. Spot illustrations slow the reader down and draws attention to itself by becoming a visual cue that can enlarge the story.
As a graphic designer, my illustrations are just that: graphic and designed. I believe illustrations have informed my every move as a designer regardless of the assignment.