I met a fashion designer once, he is a friend's godfather and I remember him making the sketches and it was impresive how easy it was for him.

This is a sketch from him.

And this is the design.

I guess that with his sketch he wanted to stand out the skirt, how big it is and the design of the fabric.

Ps: look at the back of the sketch, he used a fucsia band to make it stand up from the background, here is video for this kind of things