Just like your thoughts and ideas, sketches are in a constant state of flux, evolving and morphing as you reach a potential solution. Don’t think that you have to be able to draw in order to sketch, although having some experience with it does help.

The Theory and Practice of Sketching within the UX IndustryPaul Olyslager
5 Tips from 5 Designers to Improve Your UX SketchesUX PinFebruary 3rd, 2016
Why It’s Important to Sketch Before You WireframeUX MovementAugust 27th, 2012
The Messy Art Of UX SketchingSmashing MagazineDecember 13th, 2011
Sketches and Wireframes and Prototypes! Oh My! Creating Your Own Magical Wizard ExperienceUX MattersMay 17th, 2010
Tools for Sketching User ExperiencesUX BoothOctober 6th, 2009