I'm surprised everyone loves those pens as much as they do. Well, maybe not. It is easier after all.
You know what I'm talking about, so grab two things and begin drawing one of the two images shown below.
DO NOT begin in pencil first and DO NOT begging globally. By this I mean do not outline the whole thing and then begin.
Draw by dealing with a section at a time - including hatch and shading, and then gradually moving on. Your goal is to draw as close to the image shown as possible for a solid 14 minutes (please set your timer) knowing that you won't finish the entire thing.
Oh, and those two things you need, an iPad and a uniball pen.
Smile, be happy now.