For me the most difficult is the concept and not the design. What to draw is what people who talk to me always say.
Make some sketch on a paper sheet, and when a get the idea I scan that to illustrator.
In my opinion, in this type of illustration, the most important is the black color. Must had bold lines to give this cartoon looks. And more important is the movement, the cartoon must have movement, be graceful, like they are alive.
All of this was made with the pen tool. After the black is time to give some color and gradients. I try to use pantone colors in my illustrations because it is easy for the designer to edit my illustrations, instead of using my own colors.
Remember, first the concept and then the design. The most difficult part is thinking what is your point of view on something, and when you know that, start to draw.
At least this is in my case! :)