One most important thing about sketching I remind people are they are your personal notes to something that may become refined later on. I think a lot of people are apprehensive when it comes to sketching because what they draw doesn’t look exactly what they saw or what they imagine. Bringing and idea to completion takes time. Even the great masters took the time to plan and figure out how to get to the final piece of their work.
Sketching is a lot like note taking. You want to take as much as you can so you can go back to it for reference. You’ll most likely do more sketches after that to refine you idea. Below is a sketch of a droid I thought up quickly in my head. All the lines are merely suggestions of robotic and mechanical parts, nothing very specific, but just a quick and general idea. I can go back to this sketch to refine areas I want to later on to finalize the final idea of the droid. When sketching don’t draw the the details, make a suggestion of them, quick and fast.