You won't really know what you know until you apply it. You will likely suck at it. Doesn't matter. You'll suck a little less at it next time.
"The experienced illustrator subscribes to the principle of the application of the seat of the pants to the seat of the chair. Should inspiration whisk down your chimney, be at your table. The first ten thousand drawings are the hardest. Put another way, you have ten thousand bad drawings within and should expel them as quickly as possible."
The issue is that my passion to code is greater than my passion to design. Even if I do start at the sketch step, I all but salivate at the chance to get cracking on the CSS.
I don't suck because I can't sketch, I suck because I don't sketch. :P Whenever I do sketch my designs come out better.